Going into this kinda blind cause I dont know what engine you have. Id say youre talking about 2 different belts. The serpentine belt the only other thing to change would be an idler pulley or tensioner/ tensioner pulley which would add some expense but not that much. I'm thinking hes talking about the timing belt, if you have one. Now that one is considerably more expensive and if you get the kit which includes new water pump and usually tensioner plus maybe a few other odds and ends thats where all the money comes in. JMO but if youre doing the timing belt do the job right, get the kit, do it once and be done for another 80 thousand miles. Chances are if you don't you'll be going back later for a water pump and have to pay the labor all I over again.
