Are they trying to pull a fast one on me????

I have a 05 Chevy Cobalt that is experiencing transmission problems. When I 1st encountered the problem, I was traveleing from Atlanta to Birmingham when the car shiftedand the rpm rammed up to about 4. I took it to have a simple plug in diagnostic done to it and received codes 0700 and 0776. I took it to one shop which after 2 days still didn't do what I asked of them. I was told fine meta was being picked up by the magnet at the bottom of the transmission. Was told it would take $400 just to drop it and take a look on the inside. I told them to go ahead and do the work and a week later, nothing was done. I took it to another shop and explained all that was explained to me(wrong move). In the end, I am being told the same thing in addition to the ecu and also the o2 sensors. Total cost, $2200 to rebuild or $2400 for used trans w/ 50,000 miles and changing of ecu and o2. What is going on here? This doesn't feel right. Please hekp me see this clearly....