Very hard to estimate. Some months or years there are droughts, some weeks are excessive.
I fired a few thousand rounds out of a Dillion mini-gun in one setting. Also on that same day I fired several belts through many machine guns.
Another day I fired 400 rounds of .50 caliber through an M2, and another 400 rounds through the 240b and 249 SAW.
I'd say a totally fair estimate is 500 rounds (10 boxes of pistol ammo...) per month over the last 12 years, when I bought my first gun. That's over 70,000 rounds... or at .25 cents a round on average that's like $18,000!!! Again, these are complete guesses.