I've investigated this for two bowns and done it with one, like others, shoulder surgery drove the change.
How much weight can be taken off depends on the design of the bow. I had 7# taken off a D/R lonbow (Mike Treadway) and the cost was about $100 + shipping. The bow shoots as well as ever, which is very good. My Wes Wallace can only give up about 3#, similar cost was quoted, but I haven't given up on working up to the full draw weight just yet.
For each the bowyer is pretty much just charging the same as for refinishing, which is required. Of course, others may have a different approach.