If it is a split central unit leak may just be A-coil you could replace for about a $1000 and should include re-use old freon or new substitute freon. They have a leak sensor and can tell you and show you location of leak. Not all repairmen will want to tell you A-coil inside replacement is option. New insulated windows and added insulation in all walls and ceiling too would help in older homes. New central unit would be more efficient if you got about 10k or want to pay on it for about 5years and everything is new, may or may not be built better.
