Linuxway;
This situation comes up from time to time, and while you learn more and better cable techniques to solve these problems, there is one thing that can ease the frustration: know that it is not your fault and that you are not accountable for the unknown and unexpected--you don't have x-ray vision to see inside the wall before you agree to do the work.
I really try to have agreements (in writing, on the back of my work order forms) that mean you will be paid for your hours and are not responsible for conditions you could not possibly anticipate.
This is a BID versus Time-and-Materials debate. For some trades, like electrical work, things are more predictable. Telecom cabling, however, is definitely NOT predictable. If the client insists on a Not-to-Exceed quotation, I become very cautious that he may know of some hidden problem and wants to take advantage of the fact that I don't.
Clients (or employers) who have money, not quality and honest effort, as their main guideline, could rightly be approached with great caution.
Yours,
Mike