Is $500 to supply a wall bridge for cabinet modification a fair price?

I am a new homeowner looking to install an over-the-range microwave to replace a broken, non-vented hood. My current cabinets above the hood are 18 inches high, 30 inches wide, and 12-13 inches deep. We considered replacing these with 11x30 cabinets, but the previous owner installed custom ones, and we were quoted $500 just for a wall bridge, which we feel is too much. We live in a townhouse with limited counter space, which is why we prefer an over-the-range microwave. The existing hood is a broken circulating vent hood with no external pipe. I am exploring options for external venting, possibly through the HOA or connecting to a nearby dryer vent.