How much does the source code cost?

If it's retrofitted into the plan for a product, yes, most definitely. But that's just because of a mistake that was made before. It's like assigning to an open standard the cost of escaping from a proprietary file format.

But if it's planned ahead of time, you can significantly reduce, if not completely do away with legal review. Documentation sure may be nice to have, but we're not asking for documentation, just the source code. If you can also put in the same documentation that your own developers used, that's surely welcome, and it surely wouldn't cost you much.

As for evidence of patent infringement, that argument doesn't hold any water. If you source code could be such evidence, so does the object code, and if a third party is sufficiently suspicious of infringement, a subpoena can take care of getting the corresponding source, even if getting the source isn't actually strictly necessary.