2002 Chevrolet Cavalier Z24 oil leak

First of all, the car only has 56,000 miles on it, so I'm not happy. I have oil leaking in 2 places underneath the passenger's side, about 4 inches apart. A mechanic said he thought it was just a problem with a bolt and replaced it. I was told it might take several days for the leak to totally go away. Is this right? Anyway, it's been 4 days and the leaks are still there. I've been putting a piece of cardboard on the ground. The mechanic said if the bolt didn't fix it, it was going to be very expensive. He didn't say how much....the look on his face said it all. So, since it's still leaking I don't think it was the bolt. Given the location, can anyone take a guess at where the oil might be coming from? And, any idea at all as to how much this is going to cost me. Is it worth fixing or should I sell the car? Thank you for your help.

Louis