Here's one way to approach this: Imagine you work for a giant company where humans would push 10k lines of code per week. In a codebase like that there's no expectation that you'd understand everything. However, there is an expectation that teams contributing code will "own" it.

So if the client is contributing you should ask them if they are okay for long term maintenance and fixes of new code they are adding. If not, then maybe you should discuss pricing changes because now you are effectively maintaining code written by them which requires different set of skills and arguably higher cognitive overhead.