10% does seem a little low, without a developer you don't even get a system. If your idea is so awesome you'd just pay for a developer because you're totally confident in it.
Or perhaps you are not willing to back the idea with your own money just yet? In that case you REALLY need to up your offer to more like 40 to 50 percent in order to tempt anyone.
And also if the developer puts in all the work to make the product, whats to stop him from simply selling it independantly once he made it? Come on, at least be fair, 10% is measley, you need to show more respect.