The conversion itself can be done for relatively low cost, especially if you piece it together yourself. You can get Ranger axles and drums, retaining the rear drum on the Mustang, and on the front, use 94-95 spindles and brakes. It can be done with Lincoln parts on the front for even cheaper, but then you either have to run wheels with no center cap because of the larger dust cover on the bearings, or remove the dust cover, and risk contaminating your bearings to put the center caps on. The most expensive part of doing the swap really isn't the swap itself, it's getting a new set of wheels and tires.