hmm it can actually get pretty costly depending on what is wrong, you should check both the tensioner and idler, just make sure they are not making much noise stuff like that and the bearings seem ok, those can run you about $65 for each water pump is also recommended and that will run you about $60 belt about $40, and i say skip the alternator and serpintine belt since if those break it should be no problem to replace and you will know that they broke by charge light for alternator and loss off power steering and A/C for the other belt.
so in the end your looking at 65 for tensioner, 65 for idler, 40 for belt, 60 for pump, so 230 for the complete job or atleast or the least 40 just for the belt, but i tottaly think you should just go and do the pump at the same time ($100 total). good luck man.
BTW did anyone figure out if our cars are interference engine or non-intereference? I'm considering changing my belt but i don't want to just throw money out if i have a non-interference engine and could just go on and drive till the belt breaks.