Small Engines (Lawn Mowers, etc.): riding mower wont move, sears sears, broken spring

Question
i think i broke my step fathers mower. i dont know the model type but it is an older craftsman(5-10 yrs old). it will turn on but when i put it in gear(i tried them all)the mower does not go anywhere.everything else works. when i checked the oil it was very low and the belt under the mower appears to be on right. any ideas? sears charges  $99 to come look at it do you think it is worth paying to fix or will it cost as much as buying a new one? please any help would be great!

Answer
It sounds like it's the belt, but there's no way for me to say what it is or what it will cost without seeing it.  It could be a stripped out or broken pulley, broken spring, internal transaxle problem and so on.  You might call around to small engine shops in your area before calling Sears.  Sears tends to be a little on the high side.  At least around here.  Thanks, PK.