Why does this matter you ask? Well for a few reasons actually. I like my world to be believable and appear to be a functioning place. Travel time in this quite large empire take a long time, and I am hoping that my players will want to explore the different regions and take advantage of 'off time' activities and goals. All told, this could mean years pass in game time, so a citizen would experience all the things that should happen in that time frame in this place. Adventurers have a lot of wealth. I mean... a lot. How would a functioning government not take advantage of this? And finally, since I have thought about it, I can't unthink it. I need to know.
I even thought an interesting and funny hook for an adventure or campaign could be the tax collectors laying down the business on the player(s) to recover back-taxes on all their unclaimed dungeon loot.
My question then is this. Has anyone made any sort of system or rules or even had thoughts about this besides me? Should I just put on my comfiest straight-jacket and calm down?
If I was doing an urban campaign I'd think of stuff like this, I can picture a short story about Grey Mouser weaseling/stealing/killing his way out of taxes that are depleting his wenching and drinking funds. But in the OotA game or a regular dungeon bashing game it wouldn't add much.
An adventurer who is successful enough to disrupt the economy is an adventurer powerful enough to overthrow a government. Adventurers capable of casting Teleport and Earthquake are not people you want as enemies.
First of all you have to think of how a government can practically collect taxes (or fees).
Sales taxes like we have today are... iffy and rely upon the goodwill and honesty of the merchants. I mean if everything you do is in cash (gold) and you don't have receipts and records are hand written (duplicate books anyone?) So, imo, sales taxes will never work.
Property or resident taxes work. You can send tax collectors into an area to levy taxes once a year (or whatever interval) and simply collect from every residence and everyone they can catch, using an arcane mark or just ink to mark those who have paid. So, this works for commoners and nobles, not so good for adventurers.
Import/Export taxes work. Basically any wagon or goods that go in or out of the city gate, or go across any given boarded get assessed a tax (5%). This works somewhat better for adventurers. But of course a bag of holding can carry a lot of stuff that can easily be hid.
Income taxes don't work, because you have no way to corroborate wages or income.
But, before you go too far, what fun is this going to add to your game? It adds some chances for roleplaying, especially import/export style taxes, but will probably become boring and stale pretty quickly (again, unless it becomes part of the campaign).
That is all well and good if your players/characters are the types to do that. Being very anti-government or evil in nature. Would my players try something if they thought they could get away with it? Sure. You would just have to have established consequences in place for people attempting to over-throw the powers that be.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.