Then the question at this point is are you willing to take the risk and walk away and at what level is your confidence at in your ability to market and finish the code you currently have. You own the copyright to anything you make unless you explicitly give that right to them. I don't know what terms you signed with them however, sounds like you still retain the copyright. So I would try to negotiate them to an agreement where they can "own" the code to make whatever changes they would like (at the "own" price that they gave you), but with the caveat that they can not resell said code or offer a competing product into the marketplace. If they aren't being duplicitous this should be acceptable. If they are, well then at least you have your answer and you can base where you want to go from there (as in asking for a higher price based on potential revenues, etc). Just to give you a baseline, I've been in cases where the situation was similar and the work for building something specialized for resale was valued at $150-200 an hour (consultant hours). Just keep in mind though I have no idea what you are building and who the company is.
