The ABS light being on isn't an indication of an issue with the service brakes ; only that you have no ABS braking function .
This can be an issue with a bad wheel speed sensor , the sensor wiring or even an internal issue with the ABS control module .
Without a diagnosis , from a special read out tool , the shop can't narrow the problem down , and won't be able to provide any kind of estimate of repair cost.
Ask for an estimate on the repair as soon as the diagnosis is complete.