Buy a software – determine price

Hello,
I found a desktop based software that I am pretty interested in buying. They are offering their services for free at the moment but I have other ideas for it.
But I am not a coder or a programmer and I am not sure how much to offer. I contacted the owner and he asked me to make an offer. How should I determine the price?
Is it appropriate to post the link here?
Thanks for dropping by, appreciated.

See original post by Fisker