Historically, if you were an adventurer... You would generally cut a deal with the head of state where they would get a cut of X% in exchange for troops and supplies then, when you got a treasure hoard, you got the rest essentially tax-free and maybe even got to tax others, and would often get ennobled to boot if you weren't already if you pulled off a big enough heist, as enriching the lord's coffers by a large enough amount was often a good way into their good graces.
Likewise, in wars, looting bodies for valuables, ransoming off survivors, and then throwing the dead bodies in a ditch was kind of standard worldwide up until about the Renaissance era. Looting was considered the PRIME reason to go off to war, as it meant that one who survived long enough had a chance to become very very rich.
Letters of Marque likewise were similar in purpose to the contracts and feudal agreements made in earlier eras.
Thus, it's not stretching reality to assume that PCs, with their fantastic resources, would probably not get taxed much. 10%-20% maybe, and basically that's payment for the loan involved in supplying cash, supplies, and warriors to the adventurers. 5% if they supplied themselves, and that was payment for official protection from claimants. (usually other countries...) If it was war though, then that might get overlooked if the adventuring was part of a feudal agreement between adventurer and lord.. as going to war for the lord would be part of the 'price' for all of the rights the adventurer is given in terms of land and political power.