Gary hit it on the head as usual. Most guys I know who shoot C&B use between 20 - 35 grains, depending upon caliber. Not sure what the loadings are in BP cartridge. As I said before, that's a whole 'nother ballgame.
For some reason I can't recall exactly but IIRC there's around 7000 grains to the pound (I think). So 7000 divided by number of grains per round should equal number of shots per pound of powder.
Most guys (and gals) who shoot CAS have a relatively large budget the first year they shoot compared to the following years due to the cost of firearms and accoutrements. The outfit you wear is a bit part of CAS. We only allow someone to come in street clothes (jeans, t-shirt, ball cap, sneakers etc) on their first shoot. After that , they should at least have a modicum of western attire; a hat, western-looking shirt and boots. Some of the guys that shoot at my club have at least half a dozen complete outfits, along with several sets of firearms.
Once you're up and running though, your budget shouldn't be too bad, unless you shoot a match every weekend and travel much to get to them.