At the recommendation of people in my family who are in law enforcement and the military, I chose the .40SW over the 9mm for my first gun. I shoot about the same with either.
So, I bought my first gun, a Glock 23, and started loading up on ammo. I had been concerned about the price of ammo particularly since I need a lot of practice being a new shooter.
I just made my next ammo purchase. I'm averaging about $14 a box of 50 180gr .40SW, total shipped. When I compare that with what I see locally for 9mm (and even online), I'm only paying $2 more (accounting for tax) than what I would pay for 9mm.
My first purchase was for 1000 rounds and my latest was for another 2000. I'll have to slow down at the range since in the last month since I got my gun I've already gone through 400 rounds, but I'm not going too crazy I think with how much I'm purchasing and it's still very close in price to 9mm.
So, if I limit myself to 100 rounds at the range per visit (I'm getting a .22lr gun soon to help me waste some time there having fun so I don't shoot so much of the higher-caliber ammo), I'm only spending $4 more per range visit to be able to shoot the .40SW. I had assumed there was a huge disparity in price between the two calibers, but that's not what I'm seeing so far.
Of course, 9mm is cheaper, but it doesn't make me want to go out and get a conversion barrel or anything and start shooting 9mm.
It's still a rather expensive hobby. I still find it funny that all this money is getting shot out at a piece of paper, never to be seen again. And, I'm getting somewhat addicted to it. I'm new to it but finding it a lot of fun. I imagine at some point I'll be making my own ammo with my own reloader. But, until then I'll continue to buy in bulk and space out my range visits to help with the cost.
Just thought I'd share this with anyone trying to decide between the .40SW and the 9mm. I think the $2 a box is worth it, and if I ever change my mind I can always get a conversion barrel so I'm set either way.
Just my two cents, as a newbie.