You have to remove the external drive belts in order to do the timing belt, so replacing those external belts really(?) is no extra labor. How each shop justifies thier own labor charges is what they should explain to you.

Once the timing belt is removed, the water pump is completely exposed. Just 5 more bolts & it's off. However, that also means you have to drain the coolant. If they do a cooling-system flush & replace the thermostat, that'll explain some of the extra cost. Ask 'em for some details. Maybe the cheaper shops simply let it spill out when they take off the pump, then add make-up antifreeze, leaving your system with 1/2 old & 1/2 new...

Replacing your own timing belt isn't all that bad; but sure, it's not for beginners. Still, you could get your hands on a repair manual (library?) & read thru the procedure. That'll give you a really good idea of what it involves. Even for someone planning to DIY, they should read this BEFORE dismantling their engine...