7.62x39 brass almost may as well buy the expensisve ammo and just save the brass its so expensive.
5.45 I haven't heard of anybody reloading, or of any brass cases.
.40 is probably good bang for your buck reloading wise. I reload 9mm and save a little money, but not a ton.
If you want to save money, .223 is good and so is .308 or something. Brass is just tough to get for the Russkie calibers.