I see you've bled them dry. :-)
I am actually thinking about adding achievements, and one of them would be for taking every last gold coin from a party. Maybe it's too easy. :-)
Considering I did it to two parties yes. Firstly you can always sell at a loss to do it. Secondly the fact that characters will carry in excess of 5 potions makes it easy to finish off those small amounts. If you want an achievement it should be along the lines of >x% on a single item, bleeding a party dry with one sale (limit it to parties with more then x gold)
Seems a very high value to be paying every turn, especially with parties that don't really spend much/anything in the shop. What is the maintenance figure based on?
I like it, forces you to keep an actual gold reserve.