I know this isn't the question you asked, but if you delay finding a co-founder until after you've developed a product, then you're really looking to just hire an employee, not a co-founder. A bit pedantic, but extremely important because these days everyone wants to (a) work on their own ideas, (b) work for a super-hot well-financed startup, or (c) work for Google/FB/etc. And speaking from recent experience, I took the approach you're talking about and sort of ended up in purgatory with a reasonably-developed product but hard to get anyone to emotionally buy into it.
In any case, to your question, I was able to work with a few developers at reduced rates in exchange for equity. We simply established market rates for their service, determined how much cash I could pay them and then paid the rest in options. That was determined by using a reasonable expectation for Series A pricing and used that (you can poke holes in that as ignores the value of prefs and equity pricing doesn't really have a direct bearing on option pricing, but it's close enough and people understand it).
Jonathan's recommendation is as good as any... Fred Wilson uses one for early employees: http://www.avc.com/a_vc/2010/11/employee-equity-how-much.html.
But, again, I think you need to first answer the question of whether you want a co-founder or not.