And if you handload your own ammo, it's even less expensive.

For a high-volume shooter, it makes financial sense to handload
ammo. For example, in terms of raw materials, it costs me a little
under $5.50 to handload 100 rounds of 55 gr. 223 "plinking" ammo
for my ARs (the 75 gr. rounds used in my competition ARs cost a
little more to handload due to the pricier projectiles utilized).
In addition, handloaded ammo is also almost always more accurate,
reliable and consistent than commercial ammo.
Of course, if commercially produced ammo were available
inexpensively, it probably wouldn't be worth your while to
handload. It used to be the case that Russian military surplus
7.62x39 ammo could be purchased here for little more than a song
and a dance. However, that particular ammo is currently in short
supply (allegedly due to the war in the mideast), and prices have
spiked significantly.
(By the way, Dave, that's a nice collection you have there!)