Excellent question and the answer is that their aren't many cheap programs out their to do this. I have Sound Forge, It was bundled with with my computer when it came and it works ok. I have done a lot of searching for a good program for editing MP3 and haven't had much luck. I am sure their are better ones out there than sound forge. The problem that I always have with these programs is that they can't handle long mp3, like live shows and they usually freeze while they are processing the data(the programs usually convert the data into a wav format for editing then convert it back to mp3). I am interested to hear whats out there.
I use Acid, by the same makers as Sound Forge. It's a great program for editing and recording. You can download the demo of it, use 3.0 or 4.0, and I think the only restriction is that you can't record with the demo. And there is likely a time limit for it to function, but it should do the job.