I brought my Mini Cooper to a dealership for an inspection service. The car's front brake pads are only at 3mm thick and needs to be replaced. The part and labor cost, however, is pricey -- at $650 -- as it includes discs and sensors replacement in addition to the brake pads. Do you have any comment whether it's necessary for the discs and sensors to be replaced? And what's the reasonable price for that?

you would only need to replace the discs if they are warped or the thickness is below the legal limit. They could also have grooves and need machining (but that depends on thickness)

I would ask the dealer why they want to replace the discs. You can probably go to a Mini specialist place to have the work done and save on the labour rate.