Click this link and you will find an article on replacing the brake pads on a w124.
http://www.peachparts.com/Wikka/W124BrakePads
look at the diy article. it is really not hard. I can do the front and rear pads in my car in about an hour.
Cost - just the pads and a little bit of my time.
Pads range from like $40-$50.00 per set, up to over $100.00 per set depending ont he brand.
Give phil the partsman a call link found on the partshop or fastlane links at top of page.
With a few tools, you can do the brakes yourself. just follow the directions exactly.
Alon