Don't know how much price to offer.

So I built a web app that kinda have a buzz lately. Actually this web app is a personal project of mine that I built during my weekends and I have no experience building an app that I got paid or somebody bought it. Two days ago somebody contacted me that he likes my app very much, like the design yada yada and wants to use it on his own site. Actually he wants a little twist on it but the functionality, design and programming logic will be the same. Now he wants me to make an offer for the price. I really don't know how much to offer since haven't been in this situation before. Does anybody here experience this before ? What did you do? I don't want it too high (like unreasonably high)that might turn him away or too cheap that it will be not worth my time. Please help need your good advices. Thanks in advance.