2000 BMW Z3 Not getting an honest Estimate at shops

I have been told that my car needs front & back brakes repaired. Pads on both front & rear. Front rotors, sensors & rear rotors. Plus labor costs. I have gotten estimates of $900.00. $800.00, $706.00. I did not get an estimate from the BMW dealership, because they are outrageous. The estimates are from different shops around town in Sacramento. I don't know who to believe. I had my front brakes done back in April of 2007, & they need replacing again so soon. What should I do to find out the truth about the costs regarding this repair. I am a female & that does not help when I go in for an estimate. I just don't trust mechanics. Sorry ! Would it help if I were to take my car to AAA diagnostic center for the brake inspection. That would cost me $47.00 they told me. What do you think?