Brake Sensor problems of the 2010 Mini Cooper

I recently had my front brake pads and rotors changed on my Mini Cooper in March of 2013. Within the past couple of weeks my car's warning sensors for the brakes has gone off again. After cycling thru it was determined that the rear brakes now needed changing. I recently took the vehicle back to my service shop who generally does my repair work and they advised me that my left rear brakes were fine - no major pad wear or rotor damage - however the right side caliper was working overtime and had worn down the rear inside brake pad completey which is why the warning brake sensor was activated. My vehicle has somewhere around 48250 miles & still under the 4yr/50000 mile warranty. I shouldn't have to replace calipers at this stage. I took my vehicle to global imports (Saturday 5/25/13) to ask that vehicle be put on lift to inspect & was told after only looking up my VIN info in their computer system that it was likely time to replace brakes and how much it would costs. I asked to have vehicle looked at twice - which the service rep would not have vehicle looked at. This is a safety issue and something that should be covered by BMW/Mini. I do not drive my vehicle hard and most miles are in the city going to & from work and home. Some highway miles, when traveling. Since purchasing this vehicle from global imports atlanta in may 2010 - there have been problem after problem with this vehicle and I'm wondering if I was sold a lemon. I'm being told by my brake service guy that just to repair the right side (brakes, rotors, caliper & sensor) it will cost me around $650. 00 to fix. I dont know if this is a problem with all Mini's at this mileage or just mine - I doubt whether global will do anything about this issue but at least my concerns are documented for future Mini owner who might run into similiar issue.