I have a program that I wrote for a customer (decent sized program, billed on an hourly basis.) It was written under a no source code agreement.

Now they want the Source code.

I'm not sure what to up-charge them for the source code.

Was curious if anyone had any rules of thumb for what the additional cost should be?

10% more, double, triple no clue as to what is customary, and I'm sure it varies a bit, so I'm just looking for some ballpark ideas.

Any feedback you have would be appreciated.