If there hasn't been some problem due to the traction control system being engaged frequently, then it is far too early for the brakes to be worn out. It could be a problem with the sensor. The dealer can measure the actual pad thickness with a small tool and reaching in through the wheel, but the best way to do it is to take the wheel off and inspect it properly. My dealer's published prices in Canada for my former 2003 X5 3.0 (just as a guide) are as follows (as of May 2009):

Brake Discs & Pads (Front) $474.00

Brake Discs & Pads (Rear) $436.00

Brake Discs & Pads (Front & Rear) $893.00

Brake Pads (Front) $221.00

Brake Pads (Rear) $206.00

Brake Pads (Front & Rear) $400.00

Some posters report higher prices in the US, likely due to dealers that are trying to live up to the 'stealership' tag line. In my opinion the issue is that in the US there is no profit made by the dealer on the vehicle sales transaction, so they try and make it up on the service pricing. The above prices are similar to what an independent would charge when using OEM parts and warranting their work. I have no idea what your local service pricing should be.

Good luck.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue