it depends on the bearing. s/c pulley bearing 56$, idler pulleys replaced as a unit 20$ tensioner pulley also replaced as a unit 40$
now a/c alternator water pump bearings get into a whole nother price range!
for me when i did my c3 swap the s/c pulley bearing 2 idler bearing and tensioner puleey all showed signs of bearing wear so i replaced them all. total cost was under 150$ or so.
its just not worth risking a bearing failure and the thought of a broken pulley ping-ponging around the front of the engine just hurts!