Subframe bushings are an integral part of the rear suspension. When they are bad they put added stress on the other components and cause handling problems especially when roads are wet. I suggest you confirm they are bad and if so get them replaced. I certainly would..

Dry? They do not require lubrication except when intalled. The reason they clunk is that the metal center of the bushing has separated from the rubber that is secured to the metal center and outer metal of the bushing. For instance, if you gripped an apple around the outside so it was held together but the core was separarated, the core or center of a bushing, and then hit a bump or turned the core would travel more and 'clunk' when it hit the outer part of the apple.

is replacing the bushings a big deal? How much does the job typically go for?

I do most of the work myself so I can't give you a good estimate. I would say to find a good a good independent shop that specializes in BMWs. The job is not a big job with the right tools but even then it takes some time to press the old bushings out and the new ones in. A guess would be 3-4 labor hours--don't know indy cost per labor hour. Bushings are around $50 x 4 if you purchase on line. Purchased from the dealer will likely be more than twice the price.