The first thing I would do is take the adapter off the speedo, drive the car and make sure the pins is spinning coming out of the reverse adapter. If it isn't disconnect the reverse adapter and drive the car checking to insure that the chain (cable) is spinning. If everything is working after those checks then your speedo is bad. If you determine it is the adapter here is one for almost half the price of what you were quoted from Autometer -
90 ° REVERSING ADAPTER FOR SPEEDOMETER CABLE, 5/8"-18THREAD You can pick up a chain/cable at any auto parts store.
Good luck.