I think it depends on a lot of factors, including the timeline and efforts for rewrite, if you are going to get paid, your role and new version of product's rol

I think it depends on a lot of factors, including the timeline and efforts for rewrite, if you are going to get paid, your role and new version of product's role in new client on-boarding (expansion of client base), ... . I think you can try to come up with some projection for the near future and then value the venture based on that and then try to correlate existing or new efforts with this valuation. Leave your email and I can have a call with both of you to create a framework as I have done before for similar situations.