How much extra % of the original price do you ask for selling the full IP rights?
Some clients have been surprised by developers asking a lot of money for copyright once they want to update the app, sell licenses or raise funding - for those, including copyright transfer is a selling point for hiring me. If client's don't ask for IP rights, I bring it up myself and explain why they will need it.
I don't think having separate rates for full source code & copyright makes sense. Most of the code is not re-usable between projects, and I don't get any value from keeping it to myself. The only use of charging extra would be if I could then offer cheaper rates to people who don't want IP rights, and raise the rates for those who do: but I'm still doing just as much work for the first group.