When I press on the gas in My 1996 Grand AM, this noise, kinda like a bad muffler but not as loud, comes from by the driver's seat. Any clue to what it is or what's wrong?

sounds like a simple exhaust leak - the crack is probably on the top of the pipe so the sound is coming up through the floorboard and sounds like it's from under the seat

how much would it cost to have it fixed?

Hmmm... depending on labor rates at a certain shop - probably would be like a hour job to detect and patch it shut - anywhere from 30 to 60 $$ - i dunno I don't remember what shop rates go as usually, but I'm pretty sure it wouldn't be anymore then 100. Anyone else have any idea?