I suggest not setting the bar too high with tiered payments and such... a low bar will be more profitable ultimately because more people will be interested in using it if they don't have to take the risk of holding a bunch of QTL, which is likely to be very volatile... especially as value increases. Try to strike the right balance there.
The numbers I suggested are just place holders. But yea I definitely wouldn't want to see it too expensive. I'm glad there is an arms race to build better trading tools.