Eh. By the PHB and DMG, by the time they're 4th level, they've got more than 1,000 GP/level.
Second level is 900 gp...
So 1,000 might be too much. 500? 100?
IMHO, first-level characters could hire themselves out for 1,000 GP, and bring them to second level in their adventure and not be too out of line. Also, 1,000 GP would likely be too much to pay for a floating ball of Cure Light Wounds, and well out of the range of most 2nd-level character's GP rates. So, by the rule that you can't hire anyone more than two levels apart from you, a first level character can only be hired by a third level character, for about half is total wealth....right...
In other words, there's not much of a market for first level characters. And once you get to the low-mid range, it's another drop in the bucket, and a lot cheaper than going on the quest yourself.
Yeah, they can try to hire themselves out at first level...but they're going to be seeing the higher-level people going out, and anything they would get hired for is likely to mean their death (like I said, going by DMG guidelines, it would take at least a third level character to hire a first level adventurer...ever send a character of first level against a CR3 monster? Mush!)
And if they do survive, they'll come back with a bit more GP than a 2nd-level character, having overcome a challenge of two above their level...probably about a good reward. Probably also enough XP to raise 'em a level.
Oh, and going on an adventure is a lot more risky that guarding a caravan. It's assumed you're going to be taxed to your limit, while the wost that could happen while guarding a caravan is a bunch of orcs get uppitty...that's what warriors and adepts are for, not Fighters and Wizards! The real adventurers aren't called in until the orcs get REALLY upitty.
Guarding a caravan? Pheh. That's like standing sentry at a wall. There's a good chance nothing will happen. Delving into a dungeon to help some dudes recover the body of their lost friend who was taken by an undead horror? Yeah, something's going to happen and it's got a REAL good chance to kill you.
Remember, the D&D world is one where "adventuring" is a career choice, much like "Accounting." Adventurers are paid lots of money to do stupid things. The less mercenary may actually not try and rip you off -- most will.
Guarding a caravan isn't really Adventuring work. It's commoner work. Adventurers take it during a slow period when there's no legendary ruins to explore.
Anyhoo, 600 gp is chicken feed for that guy. He's got at least twwenty times that himself. His *gear* is worth more than that. You think he's going to go on a quest to risk life and limb when he can make just as much money selling his +1 codpiece to the local smithy for slag?
1,000 GP per level remains significant for about the time it's feasabile to hire people. The more expert people don't care so much anymore, and the less experienced are likely to be mush, anyway.
PC's aren't commoners. They're used to more wealth than commoners. Anybody with a PC class is usually considered an adventurer, making plenty of money in abandoned ruins and the like. It's commoners who form the local militia, put down a few orcs, and guard against theives. It's adventurers who lead the loacl army, wipe out an entire tribe of orcs, and are able to sabatoge an entire guild of thieves.
Ever see Jean Claude Van Damme guardin' a frickin' caravan? Ever see Bruce Lee kick butt on Joe Normal and have it be considered heroic?
Nope, these guys are going against martial arts masters, drug lords, and international terrorists.
The PC's are the Van Dammes and Bruce Lees and Schwartzenegers of their world. They may occasionally babysit for kids, a la Kindergarden Cop, may occasionally wus out for things like Junior, but, in the end, they get back to kickin' Evil Buttocks and Savin' The World.
O'course, usually the Director (the DM) makes the plots a little bit more interesting than "The clone is trying to take my life!"....but not always. There's Dopplegangers....
Anyhoo, yeah. 1,000 gp per level ain't too bad. It means that first level mooks get little work and high-level heroes don't care so much about a petty little 10,000 GP. And it means that the PC's wallets have to take a signficant dent if they want to get some adequate help in their persuit of evil.
This price is for adventurers. Not town guardsmen to protect a caravan.
