there already is: RandomByte / Taxation
But sadly the plugin is broken.
It would be really nice, if a similar plugin could be created or the existing one forked and fixed.
For me personally, I already have an idea how to use this on my server. We are planning for events at which specific buildings are build, caves are raided and so on. For these events admins have a system account. Once a day the plugin could check the current amount of currency on each players account; a) logging an entry to a database for each account known to the system with the current total, b) the current total is then compared to the total entry of the pevious day and the positive difference (which is expected to surplus) is then taxed via a configurable percentage. If the difference is negative, no taxes or a smaller tax amount is taken. All taxes are transfered to the system account.
It would also be nice, if the players which are currently online would receive a message about if they were taxed and if yes, how much.
The Taxes in the system account are then used in events by the staff to buy materials, food, water, potions for these events from the players shops. Thats at least the idea behind it.
Curreny is currently handled via pieconomy plugin and shops via CarrotShop plugin.