How much more should I charge for freelance project source code? [closed]

Question

Currently I'm working as a freelancer, and for the first time a faced a client who wants the source code too.

This client owns a software development company, and wants to have rights over the source code, but that I keep my rights too. That means that they can use my code to make similar applications using my code for different customers and the same applies for me.

How much more should I charge my client?