First, let's get the terms right.
You buy BULLETS when you are a reloader. A bullet is the part that comes out of the barel.
OTOH, YOU are looking for CARTRIDGES or AMMO.
In general, the more widespread the use of a caliber, the less it will cost at retail- which is why 12 g shotshells are a lot cheaper than .410s.
In handguns other than 22 rf, least expensive is likely 9mm Parabellum, .45 ACP, and MAYBE .38 Special.
Rifles- 7.62x54R, 8mm Mauser, 7.62x39, 7.62 NATO, 5.56, 5.45x39. Much of the cheap 30-06 is gone. The CHEAP MILSURP 6.5 Carcano, .303 Enfield, M1 Carbine, 6.5 or 7.7 Jap is LONG gone.
You can go to a site like Ammoman or Cheaper than Dirt, check their prices, or go peruse the shelves at WallyWorld.
And if you think any of THAT stuff is pricey, I have a Winchester Mdl 71. Caliber .348 Winchester. About $75/20 rounds. (John 11: verse 35)