I've seen a lot of stuff about taxes here addressing several major issues:
Knowledge of PC wealth
PCs going Outlaw
PCs vs. Law Enforcement
Realism
I'm really surprised that, at least in my reading, no one has mentioned one of the big reasons why you might not want to worry about adventuring PCs paying their "fair share" of the taxes.
If I am a PC, even a Lawful Good one, and I am expected to pay taxes on the treasure that I acquired while stopping a Drow Vampire Exarch of Orcus from invading this dimension and annihilating your town, I am ALSO going to expect to be PAID for stopping a Drow Vampire Exarch of Orcus.
I mean, if you are collecting taxes, then it is quite reasonable for your citizenry to expect you to protect them from invasion. This has, historically, been one of the sore points of tax collection. People who suffered the depradations of bandits, brigands, raiders, and invaders were often the people who complained, or even revolted, about taxes.
So, if you are collecting taxes, then YOU had better have forces equipped to deal with the undead invasion from another dimension. That would mean that the adventure consisted of watching the duke's army foil the invasion.
Oh, wait. The Duke can't stop the invasion of Undead? Is he going to PAY us to rescue the town? Well, if we are paid appropriately (what's the going rate for stopping a demonic incursion?), then I don't suppose I really have any problem with paying some taxes on my income.
Oh, he wants us to volunteer to face death and destruction? Oh, so we are doing this for free, AND the duke levies a tax on anything of value we happen to find on the people trying to kill us? Why exactly are we protecting this guy's right to rule?
While I am sure that you can find real life examples of people who call the volunteer fire department when their house is on fire, and then try to bill the volunteer fire department for the water damage their house sustained, I assure you that such people are @$$hats.
If the local lord is such an @$$hat, I think either your players are going to realize that you want to take money away from them and acquiese, or their characters might seriously consider deposing such a ruler.
The ruler is unable to protect his people, unwilling to pay people to do so, and intent on financially penalizing anyone willing to do so for no pay. He's automatically NOT a good ruler, and his actions have jeopardized his people.
Now, if you want PCs to pay taxes on goods, I would assume those are already factored into the prices in the rule books. If they want to own land, raise crops, sell goods, provide services, those things are all not covered in the books, and totally up to your group ANYWAY.