Y'know, one of the biggest downsides I consistently hear about .357 SIG is the cost of ammo. Is it as cheap and plentiful as 9mm? No, but then most major calibers aren't.
Here's a quick look at .357 SIG prices compared to .45 ACP.
One case of Prvi Partizan .357 SIG comes in at $359 from Aimsurplus. Since Aim has nothing comparable in .45 I'll go to Palmetto State Armory, where a case of PMC .45 ACP is the exact same price at $359. Georgia-Arms sells Speer Frangible for $300 and new .357 SIG for $310 Comparable Georgia Arms factory new .45 ACP is $400.
.40 S&W is just slightly cheaper, but then in my experience it's the cheapest common caliber of those above 9mm. Furthermore, virtually any gun chambered in .357 SIG will also shoot .40 S&W, so you have the added benefits of both cost savings and versatility if you buy a pistol chambered in either caliber.
I actually found .357 SIG both cheaper and significantly easier to find than .45 ACP during the Great Obama Ammo Rush. .357 SIG is certainly more expensive than most other major pistol calibers if you're buying it by the box, but if you buy it by the case or half case the price difference is negligible.
Besides, it's just a damn fun round to shoot!