Galactic Marketplace

Owning the Marketplace on the Galaxy will provide unique bonuses to the owner:

• Cash and Set transactions taxes: as each transaction made on the Marketplace will be subject to taxes, the owner will be able to set them, at a rate varying from 0% (triggers a global notification to all empires) to 25%

The Problem:  There is currently no reason not to set it to 25% tax rate.  I want the most money from everyone so tax tax tax!  

Perhaps there should be a maximum hike or decrease in taxes that can be made and locked in for a certain amount of game turns.

There is no penalty (that I see) from increasing the tax rate.  Each percentage raised should have some negative consequence on the controlling  empire.  Perhaps a Happiness hit?

I don't see anything wrong with an (say) additive negative 1% hit to happiness per raise in tax rate over 0% (your empire only  - why shouldn't the population rebel against higher tax rates).  I find I'm swimming in happiness by mid to late game.

Not to mention the cost involved to raise taxes or lower them is next to nothing.  

There should even perhaps be an influence cost added in order to change tax rates (the higher the tax rate increment, the more influence required).  Or an influence cost to maintain a certain tax rate.