There are a lot of quality leather shops out there turning out rigs for cowboy action shooting. In general, you get what you pay for, but some are more expensive than others for comparable quality. Even at the more expensive makers, though, the price usually varies a lot depending on the style and how much ornamentation (carving, conchos, etc.) you get.
Your best bet is to order a basic, no-frills rig from a reputable maker. I think one of the best values is
Circle KB's "Cowboy" unlined rig. A rig with ranger style belt, cartridge loops and your choice of dual straight draw (duelist style) or strong side/crossdraw lists for $249. It's made of heavy leather that holds its shape and will last for a long time. I know -- my first ever cowboy action rig was a Circle KB Cowboy Dualist, and I still use it. I've bought other rigs that cost more and haven't lasted even a full season. Circle KB is also a small shop run by the nicest folks ever. They'll make sure you're happy.
Other makers I can recommend are (in no particular order):
Black Hills Leather. Rudy Lozano is the owner and chief craftsman. He makes some of the finest metal-lined fast draw rigs in existence. He also makes excellent CAS rigs. His leather is absolute top quality, but his prices also tend to reflect that reality. Fortunately for you, he has his annual end-of-summer 20% off sale going on right now.
Old West Reproductions . Master craftsman Rick Bachman has a different philosophy from the others. He doesn't focus on designing competition rigs. Instead, he makes highly authentic copies of real rigs that were used by real cowboys in the real old west. He does extensive research and has one of the world's finest collections of historical gunleather. If you get an OWR rig, you know you have something authentic.
Kirkpatrick Leather. Mike Kirkpatrick runs what is probably the biggest leather shop serving the cowboy action market. He makes fine quality competitive rigs, many of which are named for top cowboy action competitors (a lot of endorsement deals going on here). His rigs are moderate to expensive, but they are very high quality.
San Pedro Saddlery. San Pedro has an almost cult-like following on the CAS circuit. Craftsman "Big Ed" does very fine engraving, and is known for his use of pigskin lining (which is beautiful and pampers your guns, though it also tends to be a little "grabby" for my tastes. Big Ed's rigs are almost as common on the CAS circuit as Mike Kirkpatrick's.
Links to all of the above makers are here:
http://www.circlekb.com
http://www.blackhillsleather.com/
http://www.oldwestreproductions.com/Splash.asp
http://www.kirkpatrickleather.com/
http://www.sanpedrosaddlery.com/
For all of them, figure on 6-10 weeks from order to delivery, as all rigs are made to order. You need to specify the style, color, number and caliber of bullet loops, holster types, gun types and barrel lengths, etc. It's best to order over the phone, where you can talk to them and get advice. If you don't know how to measure for a gunbelt, they can help you (for most men, the size to order is 4" larger than your pant size, but that doesn't always hold true).
Good luck!