The treasure tax was not a fixed rate.
It varied from 20% to 60%.
It was determined on a case-by-case basis. The rate changed frequently and fluctuated wildly. It might be 30% today and skyrocket to 60% tomorrow, making it impossible to deduce its underlying principles or predict its trends.
If one didn't have enough funds to pay the "treasure tax," they could put their treasures up for sale on Treasure Star. Regardless of the sale price, after the transaction was settled, the fixed daily "treasure tax" would be deducted, and the rest would belong to the adventurer.
