Brass availability can be gaged by its popularity, having about 5 categories, each representing a successively higher price range.
1. Military/Police Surplus.
Usually around 10 cents per case (or less) for new production brass, the 223 is the most abundant rifle case, and so is the least expensive. 22 Hornet brass is in the same price range, but the smallish size of the case is probably a contributing factor.
They sit in the same price range as the common handgun brass for 9mm, 38 Special, and 357 Mag.
These calibers (except for 22 Hornet) are so widely used, the fully loaded ammo can be found at less expense than purchasing the individual components. So the brass is commonly available after being fired once for an even cheaper price, from 1-5 cents apiece. That's cheap by today's standards, and they may need to have the bases swagged in order to remove the military crimp.
2, Common Sporting Calibers.
This category includes all the common hunting calibers that are chambered in the majority of factory rifles, such as 22-250, 30-30, 243, 270, 308, and 30-06. Usually found between 10-20 cents per case.
3. Less Popular Hunting Calibers, Wildcats, and Some Target Calibers
Strangely enough, even though the only difference between a 308 and a 7-08 case is the neck diameter, the 7-08 fits into this category. But it sits with some other well-known calibers: 204, 222, 222 Magnum, 25-06, the wildcats based on Mauser cases, the new RUMs, SAUMs, WSMs, and WSSMs, the Remington Bench Rest calibers, many of the Ackley Improved cases, and the standard magnum chamberings including the 300 Weatherby. Price is typically within the range of 20-35 cents per case.
They are also found in various grades of quality that only a regular shooter would appreciate paying a higher price for.
4. Independent Factory Endorsed Wildcats
Typically the PPCs, and any specialty caliber that companies like Kimber, EAB, Bulberry, H&S Precision, Weatherby, and many others would be pushing as a regular chambered caliber for their own products. The price often exceeds $2 per shell in price.
5. The Obsolete and Rarely Used Calibers.
Fortunately the brass companies keep some of these alive. I think the highest price I have seen was around $10 per case, but understandably, some brass can demand a considerably higher price.