2002 Chevy Suburban Air Conditioning

Let me start by saying that I have no idea what the engine size is in my car. My husband is not here right now and I came across this site because I my car in in the garage and I need some advice.

I had my car into the Monro muffler place about 4 weeks ago and they charges my airconditioner and put in freon. About a week later, I had no airconditioning. I took it back and they did it again and this time, they added a new O ring. It worked for about a week and then I noticed that it kept getting warmer and warmer until it didn't work anymore. So today I took it to the Chevy garage. They said it could cost between 200 - 2000 dollars. I am obviously hoping for the lower end. I just wanted to be able to understand what is happening. My thought is that their is a leak somewhere in the system because it does work and it slowly went away. That seems like it shouldn't cost too much to fix. The gentleman at the garage said that it could be the compressor and that that will be costly. My question is: Could it really be the compressor even if it works for awhile? It seems to me like if it was the compressor that it wouldn't work at all. Help?