@Connington Thank you for that laugh, and it brings up a good point. We can't think of taxation in the traditional sense in terms of applying to adventurers. In a game I am in now, the Duke of the area actually formed an office, the Office of Adventuring and Dungeoneering (it's basically the DMV), which specifically deals with both licensing adventurers, and handling adventuring contracts. All adventurers have to be licensed (which includes a fee), and in order to complete the licensing process they have to join an adventuring guild. Each guild has to have a guild hall and lodging for members (which it pays property taxes on), as well as paying a fee to remain an official adventuring guild. In turn, the guilds make their money by taking some of the spoils of their members, when their members complete contracts given by the guild. If a person wants to hire adventurers, they go to the OAD, which in turn gives the contract to a guild (it seems like it is only one guild that gets each contract, so presumably they bid over them), and then the guild members can find the contracts in the guild halls. Adventurers accept the contract, go do their thing, keep the loot and also get a fee for the contract (which the OAD actually pays half of, since the Duke wants people to be willing to hire adventurers if they are in trouble).

My point is, trying to tax adventurers directly would probably be difficult to implement and probably pointless, much like it would have been pointless for feudal lords to try to take taxes from each individual person, rather than simply taking a share of the harvest.