Cheapest I can find (for target practice, of course).
Main ammo I'm acquiring is Yugoslav 1952 7.92mm military rounds for my Mauser. A friend just found 1954 rounds which shoot a little bit better -- 2-3in groupings at 100 yards with iron sights and a crude rifle rest; consistent hits on a 2ft target at 200 yards, again with iron sights.
For my 9mm, I bought a "value pack" of Winchester rounds. Now I know why they are a "value pack" -- they smoke more than a muzzle loader! But, $10 for 100 rounds for use on the range is just fine with me.