7.62x25mm Tokarev and 8mm Mauser are currently cheap to obtain, but the availability of inexpensive decent-quality milsurp ammo in both of these calibers is becoming spotty.
Keep in mind that 7.5x54mm French MAS and .303 British were considered cheap milsurp cartridges for years, but then the supply of quality milsurp ammo dried up, and many of the guns have been relegated to owners' safes. New-made factory ammo in 7.62x25 will probably never be as cheap as other common pistol calibers due to the complex and unusual bottleneck case; keep this in mind.
FWIW 7.5x55mm Swiss milsurp ammo is not as cheap as the others, but the cartridge has a big advantage- it uses the same .308"-caliber bullets as common .30-caliber cartridges such as 7.62x51 and .30-06, making it much easier to handload once the milsurp stockpiles are gone.
My next major C&R purchase may be a Walther P38 or a Luger P.08; the guns themselves aren't cheap, but I can shoot
them for cheap because I've started handloading 9mm Luger in bulk.