Brakes/ ABS light on a 98 Cavy.

I've posted this before. But I took my 98 Cavy to have a slight grinding noise, not to mention some sliding issues when I tried to park at very low speeds. Mechanic said it was an ABS speed sensor, that + cleaning and adjusting rear brakes=$224.00. The same symptoms are appearing again. I fear this might be an extremely expensive problem. Any thoughts or theories would be appreciated.