Ask HN: Buyout cofounder for iphone app

Does anyone have any guidance for pricing an iPhone app? I worked on one with a friend for a couple weeks with the agreement of splitting anything 50-50, and it is almost making enough to cover a single person's living expenses. I think I can grow it more and would like to keep working on it, while my friend isn't interested at all, but still wants 50% of whatever growth that occurs. I'm hoping we can come to some agreement for me buying his share in it.

Any advice for how to determine such a valuation? I am thinking we should determine what profits we can expect each month for the next year or so, and then apply some discount rate to it. Of course tweaking those parameters can change the final valuation a lot, but wondering if people know some ballpark figures or if this is the wrong approach entirely.