2004 Pontiac Vibe squeak/wine - engine at idle

For the past 40,000 miles, I have had a noise problem. I had it in to the dealer several times but he could not duplicate it (or did not want to). After warranty, I had the serpentine belt repaired and thought it had gone away. It came back. As I was looking on a used car lot for a second car, I talked with a mechanic there about the noise. He though it might be the Tensioner (spelling) the keeps the belt steady. I put up with the noise for another 5,000 miles until my wife said: "enough". Almost at the same time, my oxygen sensor put my check engine light on. I asked around and found a highly marked mechanic (who turned out to be a high-school friend of my wife's). So, we took the vehicle there. With an oil change and the above repairs, the total came to $850. The noise is still there and I've had it back in to them 4 times. Here is my questions:

#1 Is the cost too high for these things (I think the oxygen sensor was like $225 just for the part)

#2 Do you have a suggestion of where I can stop this squeak? (Kinda sounds like a cat. Only seem to happen at a stop light or stop sign or when the motor has a little strain on it. Say, pulling out of the driveway - turning and putting a strain on the motor/body.

Help!

WF