If you're good enough to charge money for DMing, you're good enough to charge real money for DMing. Don't bother with a tip jar - all it will do is stir up (potential) hostility, and won't make enough money to be worth your while.
I have given the matter of paid DMing a fair amount of thought, and have posted about it before, so what follows is a discussion of where I think people might pay for a DM, what I would offer, and what I would charge. I'll note that the play the bagpipes, and the scenarios are generally analogous to how I might charge for various services there. So, there is some basis in reality here, but it may not work out exactly the same in practice.
The very first thing to consider is: do you really want to be paid for DMing? It sounds a strange question, but the thing is that both DMing and playing the pipes are things that I do "for fun". I approach them as such, and am basically free to put in as much or as little effort as I see fit. If, however, I'm being paid then they're not "for fun", they're business, and I have to approach them differently. I can't just decide to skip a week, or put in a half shift, or not bother preparing, because I'm being paid. It's just something to consider.
Now, having considered that, is there a market? In general, I don't think there is. Most groups play with friends, and friends generally don't charge friends. And there tend to be enough DMs out there willing to run games for free as to make paid DMing a non-starter. That said, there are three scenarios where I think there might be a market:
The first is a group of players who want a really good campaign, but none of whom want to go to the trouble of running it. If they have enough money, they might consider having someone run it for them... and there is a certain perception that a paid DM is likely to be better than a non-paid DM, so they might choose such a DM over a free one for that reason. However, I consider this the least likely scenario.
The second is the 'special event' DM. Perhaps a group of friends used to play in high school, but they went their separate ways and the group split up. Now, Bob's fortieth is coming up, and the group are getting together for old times' sake. But no-one has played in years, and no-one really wants to run the game. Enter our professional DM... I consider this the most likely scenario.
Finally, there is the convention DM. Given the budgets on which most conventions are run, I don't see much opportunity, but if a con organiser wanted to make absolutely sure he had DMs, or if a game store wanted to make sure of a DM or two for D&D Gameday, there might be an opportunity there.
The services I would offer, and the rates I would charge, would depend on which scenario we fall into.
The first scenario I see as roughly analogous to being paid to tutor someone's child in playing the pipes. The instructor shows up, does his thing, and then leaves. In this case, I would offer a full campaign. The players could basically choose the system, supplements and house rules, subject to my actually being able to run those things (I couldn't, for example, offer Paranoia, or the use of Magic of Incarnum, because I don't have access to, or experience of, the material in question). Obviously, if we can't come to agreement, we go our separate ways, which is fair enough.
One thing I would insist on is that, once the rules, house rules and supplements are fixed, it is for the DM to adjudicate the rules. In the same way that I won't teach a child who regularly throws tantrums, or won't practice, so too will I not run for players who refuse to accept that their character just died. If it comes to it, I'll leave and not return.
For that sort of a game, I would charge £12 per hour, both for play time and prep time. If I can't charge prep time separately, make that £18 per hour of play time. (This assumes I'm not using a pregen campaign setting and a pregen adventure path. In this case, reduce the latter figure to £15.) That is what I would want paid - the group can have one guy pay, or can split the money, or whatever. I want paid in advance, at the start of the session, and if you cancel with less than 24 hours notice, then I still want paid. Oh, and in case it matters, any campaign setting and adventure material generated remain mine. PCs and associated IP belong to the players, and the storyline of the campaign is owned jointly.
(A note on exchange rates: £12 is currently equal to $24, but that's more a factor of an unusually weak dollar at the moment. If I were doing the same in the US, I suspect $16 - 18 is a more realistic figure. Not that I would expect anyone to actually pay those rates. Oh, and also: this assumes paid DMing isn't my main source of income. If I'm booking 40+ hours a week, then it's my main job, and the rates can come down slightly. As it is, there's a premium for cutting into my leisure time.)
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The second scenario (the 'special event' DM) I see as being similar to hiring a piper for a wedding or other event. In this case, I would ideally prefer to meet at least one of the participants in advance in order to discuss expectations and any special requests. However, for the most part, the players get to specify the system (and perhaps level), but that's about it. The game will then be run from the core rules of the chosen system, and without house rules (or perhaps a few simple and streamlining rules, such as "no encumberance").
For this scenario, I will provide absolutely everything that is required for play: rulebooks, character sheets, miniatures, battlemaps, dice (not snacks or a venue, though). Unless the players have specifically requested the use of their own old characters, we use pregens of my design. And, in case it matters, all IP related to the game remains mine. The rates I charge are based, in part, on reuse of the same characters and adventures, so while players can keep their character sheets, they don't own the [/i]characters[/i] themselves.
Rates: £100 for half a day, more if I have to travel a significant distance (and, if I have to stay in a hotel overnight, then they're paying for that, too. Not that I think that's likely). Payment, as always, is in advance, and if you cancel with less than 24 hours notice, I still want paid. (That's probably $150 if I'm doing this in the US.)
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Finally, there is the third scenario (the convention DM). This is a blend of the two options: I'm likely running several groups through the same adventure, using a specified system, pregen characters, and so forth. Again, here, I'll provide everything for play, and retain IP rights (unless I'm using someone else's material, of course). And I'll charge the same £12 per hour as for the first scenario.
The peculiarity of this scenario is that it's the event organiser who pays me, not the players. If the convention want to charge customers as they come in the door, that's up to them. But I'm not going to chase some kid who sits down at my table for his share of the table fee. Again, payment is in advance.
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You're probably thinking that these rates are extremely high. I'll note a couple of things:
1) The customer is paying for my time, and that's what my time is worth. That I'm DMing is largely incidental - that's the reason they're willing to pay, and it's the service that I'm providing, but it is my time that you're paying for.
2) As noted above, there's a premium being added because this is my leisure time that I'm giving up.
Additionally, it is worth noting that if I'm being paid to DM, then I'm approaching it as a business, and not just as a hobby. As a consequence of this, I will arrive at the appointed location slightly in advance of the specified start time. I will be clean, sober and appropriately dressed, will have everything I need for the game, will be properly prepared, will run the game to the best of my ability, and will basically do everything in my power to ensure that all present have a good time. The guys I run games for "for fun" get none of these guarantees.
In closing, I'll note that I have never actually charged for DMing. At length, I decided I'd rather keep it "for fun" rather than deal with the hassles of making a business out of it. (And, besides, I don't think there's much of a market there.) YMMV, of course.
