About 50% of the sites total is IMO a fair price for source code and original images.
However, you still face the possibility of them arking about in the source code.
I've sold my source code 3 or 4 times, and one thing I have learnt:
I now sign a contract on transfer, saying I pass on the rights to any code contained to said company. That at the point of transfer, I (or my team) are no longer held liable for any of said code and support for said code will cease.
Any work on the site/code/images from said point would be classed as a new business agreement between the two parties as we cant be held respoionsible for changes have made by outside influences.
I'd also make them move the site from your umbrella account at TSO.. or include a free transfer elsewhere "in your price".