Honestly, if it would take you a couple months to implement it full time and they think they can get it for $7k otherwise its probably not a realistic estimate

Honestly, if it would take you a couple months to implement it full time and they think they can get it for $7k otherwise its probably not a realistic estimate on their part and you'll never be respected for what you brought to the table. An idea is not worth 80% and making it come to life certainly isn't worth 20% if you truly want to do a startup you have a community right here to find friends and make connections. You can set up a repository and both work remotely part time to see if it works out. My point is mainly this, you are the only thing stopping you from doing a startup, so just do it. But their idea (even if it's a good one) is not the only one that is a feasible startup so why work on it when you know going into it youre going to be under appreciated from day one. Just my $.02