If you're currently shooting 100 rounds a month of 9mm para, let's look at what you'd save:
Brass - basically good for 10-12 reloads in 9mm para, perhaps a bit less as the little buggers get lost in a semi-auto. 9mm para brass is cheap as there's lots of it. Plan on $12 per 500, or 2.4 cents per piece. Even if you only get 6 loads out of it, it's under a half cent per shot.
Bullets - you can get copper plated 124 gr bullets for about $41 per 500, and hard cast bullets can be had for about $32 per 500.
Primers are about $30 per box of 1000.
You'll need maybe 5.5 to 6 grains of powder per shot. Assuming 6.0 grains, with 7000 grains per pound and $25 per pound, each shot will cost you 2.1 cents.
So you've got .4 cents for brass, 2.1 cents for powder, 3 cents for the primer and between 6.4 and 8.2 cents for the bullet, for a total of 11.9 and 13.7 cents per round, which scales put to $5.95 and $6.85 per box of 50 respectively.
If you want a good self defense bullet add another 10 cents per round and $5 per box of 50.
On the other hand, 9mm is everywhere and the prices are first cheap as pistol ammo goes, so it's a bit a buyer market if you're not picky about what you are buying:
You can find commercial ammo for around 14 or 15 cents per round if you don't mind the steel cased communist block ammo.
You can find factory remanufactured ammo with plated bullets or FMJs for around 17 cents per round.
You can find Federal 124 gr FMJ ammo for around 22 cents per round.
The only place you start saving money is if you're loading premium bullets to produce premium ammo. That's where you start to save some serious money with 9mm ammo.
On the other, other hand however, if you load your own you've got full control over the load and the quality and you can tailor the load to your specific needs. That can be handy.
I have a Uzi carbine for example and people seem to love them or hate them. A large part of that depends on the condition they are in (if the cover drags on the bolt they'll mis feed - you want .010" to .015" clearance). However a big part of it is having a load that generates enough recoil to get them to cycle properly, and not many of the low cost loads will do that. Load my own +p velocity loads however and it'll run like a swiss watch, even with inexpensive plated bullets, and with a reflex sight on top it'll produce 4-5" accuracy at 100 yards. Great fun on steel plates.
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So with 9mm para you won't save the kind of money you can with other pistol rounds, like .357 Magnum which seems to see for $30 a box even in cheap promo loads, but it may still be worth it if you like having more control over quality, consistency, and availability.