I've decided to offer the 7950 for now. The problem with charging 10-25% per month is that because this a start up I would have to charge users for at least 4 months up front.
Price ~ example?
It would cost £249 or $369 for the 4 months so around £65 or $99 per month. I would also offer a dual GPU bundle for £440 or $675 so around £110 per month or $165. I'm also thinking about doing an extreme rental option by offering the 7990 for rental. The 7990 would cost £149 or $250 per month with payment up front for 6 months.