I used to participate quite a bit. I even went to Camp Perry one year. As posted, it is expensive if you get real serious about it. The rifle is the easy part. You will definitely need a coat, a spotting scope and stand, a mat, a cart, a glove, tons of little accessories, and pretty good eyesight.
AR-15s are cheaper than Garands and M-14s to shoot, and are, on average, more accurate and stay more accurate longer. Nearly everyone who is serious is shooting ARs, but Garands and M-14s are still a lot of fun.
Before you jump in whole hog, I strongly suggest hanging out at nationalmatch.com to help you select the right gear the first time. I have been really happy with Creedmoore for my coat, Kowa for my scope, John Hollinger's White Oak Precision for rifle uppers and parts, and would probably get my next scope stand from Giraud tool. It all adds up to some cash, but "buy once, cry once" really applies in high power. Ask me how I know....
You may want to shoot John C. Garand matches with a Garand first. You can start out with less cash outlay, and they are a lot more fun to me. A bit more laid back, and everyone should have a Garand anyway!
Oh, you'll need to get into reloading sooner or later. Factory ammo is just too pricey. Reloading soon became more fun than shooting to me, and you get a lot of satisfaction seeing your reloads shoot well.
Ddurango