@Hell_Satan:
What you mean is mostly for servs which were nothing short of slaves to their lords - and I mean that for real, they belonged to their lord.
They were pretty much forced to give all to their lord and only left with a minimum to live - sometimes not even enough for that.
The farmers who were "free" also paid high amounts for "rent" to the lord and one tenth to the church as tax.
But that would still leave them more than with 10% because 10% would surely not be enough to sustain them.
Furthermore rich or influential people could even get a tax reduction or exemption by using their influence and money.
For example rich merchants most likely bought themselves such exemptions / reductions from their lords.
As for why that worked - probably because the lords found it better to have a guaranteed amount then take the risk of the merchant not doing well later on or leaving their territory completely due not getting such reductions / exemptions.
But what we got here in the story is neither of that to begin with.
You must think of what monsters do.
They endanger the safety of the people, hence the income of the lord by that.
If a monster attacks a human settlement or even just the farmland it causes destruction and deaths.
That means a monster is something comparable to a bandit.
A strong and dangerous monster would as such equal a bandit with a reward on his head.
So subjugating a dangerous monster should be the same as subjugating a wanted bandit and hence grant a reward.
For a bandit you would get the bounty and for a dangerous monster there should either be a reward, too, or the benefit of selling the raw materials with at least low taxes on it.
Let me put it this way:
If there are dangerous monsters and you subjugate one but then have to pay 90% of the earnings that you risked your life for taken from you would you subjugate a monster in that lord's territory again?
No, you would leave and subjugate monsters in places where you get to keep the majority of the earnings.
While lords and governments want money they also want to save expenses they needed to pay to do the subjugation themselves - including paying for failures.
Therefore if there is a capable monster hunter in their territory who hunts the dangerous monsters without them having to pay him for it with money from their pockets would be something they would welcome.
Hence they would try to keep such a monster hunter in their territory as it saves them time (they do not need to bother with the matter of subjugating monsters) and money (expenses for food, soldiers, weapons, etc.).
That is why taxes on sales of monster parts should be low for the people that hunt them.
At least I think that would be the common sense for a world with monsters.