At my shop we mount for $40 and if you buy a boot, binding or ski it is half price. This is actually very inexpensive, after you consider the cost of insurance, mounting jigs, and release testing equipment.
All my local shops said $100 because they needed drilling. one place explained it was so expensive because some shops had been having problems with skis where the top/graphic layer didn't match up perfectly so the marked "ski centre" was wrong so the bindings would be placed wrong. those guys then did mine for 30 bucks and mounted them like 10cm back from the ski centre when i said i wanted them centred about a 100 times