You make an offer, he can either take it OR he can pay you that price and buy you out of the business. Sign an agreement to that effect before you make the offer.
Thus, you will offer enough that if he decides to buy you out, you'll be satisfied with the amount. This is the fair price. You cut the cake, he gets to choose which half to take.
this sounds like a fair approach - a shotgun clause right? i feel emotionally invested in the product since it was my idea, but he put a bunch of time into it and we both believe in it.
we've both contributed equal time to it, using our different skills. he's written more of the code, but the designs are mostly mine, and i've done all the research into advertising, CTR, managed our intern, and done the biz admin.
Thus, you will offer enough that if he decides to buy you out, you'll be satisfied with the amount. This is the fair price. You cut the cake, he gets to choose which half to take.