Ask HN: Help with counter offer for a stake in company

Hey guys, I have been approached by a friend with a startup idea. He has no technical skills but he has marketing skills and he has resources, both financial and potential customer wise. Project itself can be built in a few months with a ballpark of $30-40k.This is not the first time I have been approached by people with startup ideas. Actually, I have relatively good experience at finding teams overseas and managing / building technology with them as I am a developer as well. I delivered a few projects before with this approach. However, I have always quoted the cost of project to those who wanted to get it built and didn't pursue an ownership. I am considering taking a different route this time and considering doing a counter offer and ask stake in the company for part of my services instead. This is the part where I am clueless. Is there some written or accepted formula for technical people like me, so we can at least calculate the percentage we should ask in these type of situations. There's definitely some hard cost for outsourcing parts of the project and things like servers etc as you can imagine. I will clearly won't pay any of it. ideally I would like to get paid to be able to feed my self and have some cushion too should things go sour and it fails. How should oneself approach to these kinds of situations? I would really appreciate any feedback.