Robodog has highlighted the correct method, although I think $60 is probably a bit steep. I would be tempted to say 40-50$. That being said, you have to take into account how much the code is 'worth' to the company involved.

You should do a cost analysis of how much the application will save/make the company and then assess what percentage of that value you think you can take away from them.

For instance, if it will make the company $500 and save them $400 that is a total of $900 - in this instance you will see that a straight cost of $60/hour is not an acceptable charge, and indeed you will have to bring the proce down - but a small percentage of $900 is a lot better than asking too much and getting nothing.

Hope this helps.