A fixed bid is a fixed bid. There is no cost increase or decrease in the amount you pay unless you add on something not in the original contract. If all you're paying is the original amount, and you're getting is the original scope of work, and you were OK with the amount you contracted for, then it's none of your business what the cost were that the contractor incurred from his subs. That's his business. There may be something shady going on for tax purposes etc., but you are not involved in it, so stay out of it.

If on the other hand, this was a time and materials contract, you have serious fraud going on.

Live wire is correct. Lets look at another scenerio. You sign a contract for a fixed amount. The job starts, but one of the Subs back out. I have to scramble to get another price. It turns out higher then what I was quoted originally. Would you pay me for the higher cost?

Also, sometimes the GC will shop to get a better price from other Subs after the contract is signed to save money.

On a cost plus job, we show the client everything. Materials, Labor and Sub costs, down to the penny.