What to charge for a ready-built piece of software? [closed]

Question

Recently I was contacted by someone interested buying a piece of software that I wrote to automate a process. The 'client' knows little about the program, except what it outputs and how easy it is to use. They are now interested in purchasing it, and I need to give a reasonable price.

I've never done anything like this before, so as you might be able to guess, I do not know where to start with a price. I spent a good 12 hours+ working on the program itself. The client will probably want some changes also.

My question: How can I work out what to charge for a piece of software that I built in my own time?