I'd be surprised if ammo price figured in this at all. Most hunters are either using "hunting grade" off the shelf stuff or reloaded ammo. When it comes to either the price between the .223 and .22-250 is very small. Only time .223 is dramatically cheaper is with the crap, wanna-be milsurp stuff from overseas.
The .223 is a good round but I'd bet much of it's popularity comes for the guns and actions it's chambered in.
CB