How much of the code comes from a framework or code that you've used before compared to the amount that was custom written for the customer?

>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.
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>Now they want the Source code.
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>I'm not sure what to up-charge them for the source code.
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>Was curious if anyone had any rules of thumb for what the additional cost should be?
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>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.
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>Any feedback you have would be appreciated.