Mazda 3 2007: Brake problems

I'm shocked at the problems we've run into with the brakes on my daughter's 2007 Mazda 3. Recently its brakes failed and the pedal went to the floor. Our mechanic replaced the master cylinder, but they failed again. The mechanic replaced the master cylinder a second time, but again the brakes failed. He then sent it to a specialist who diagnosed a faulty ABS module. The repair costs were quoted at over $5000. We're already hundreds of dollars out of pocket and this seems to be ridiculously expensive. Mazda didn't seem very interested since it was out of warranty. I accept that wear and tear items like fluid, pads and rotors need to be replaced when worn, but surely the ABS module should last more than seven years. Have you heard of this problem and is there a non-genuine part that's more reasonably priced?