If it is an 1845 model Colt, and is actually .45 caliber, it may be exceedingly rare, since there were .44 caliber Colt Cap & Ball revolvers (Walker and Dragoon) models in that approximate time frame, but I have never heard of an " 1845 .45 caliber cap & ball Colt" .... Am not sure there ever was such a critter. To the best of my knowledge, Colt never made a .45 caliber revolver until the advent of the Single Action Army in .45 Colt caliber in 1873, and this one was a cartridge arm, not C & B !!

IF this gun has truly been represented to your brother as an " 1845 Colt Cap & Ball revolver, in .45 caliber" , have it checked by an expert for authenticity before parting with any cash!! There are a lot of fakes and forgeries in the Colt field from that era!!