Fee arrangement for programming

I'm partnering with a programmer to produce an application that I'm going to sell. It was my idea, I'll handle the marketing and business end, and he's going to write and maintain the app. It's not a big program--might take him several weeks if he did it full time. In lieu of cash up front, he's agreed to take a percentage on sales. My question is, what's a typical or customary split on revenue for arrangements like this? Or if no one here is familiar with it, is there someplace else I might be able to find out?

Thanks.