Most garages charge about $400 because it's labor intensive. A new engine costs a shload more than that if you're paying a mechanic to do the work, and that's what you'll need if it goes.

IF you know somebody that can do the work, you can get all the parts for under $150. You need a belt, water pump, tensioner pulley, and new accessory belts. You'd better pay that person for blowing up their day. It will no doubt take them a whole day to do it if they don't have all of the tools mechanics have.

Taking the upper timing cover off and inspecting the belt for cracks on several different spots is one way to check it. The other test should involve wiggling the belt. If it's loose and has more than an inch of deflection... you're playing with fire. It could jump and your pistons could shake hands with the valves...