Personally I stay away from those memberships because they are so expensive and the member campgrounds may not be where I want to go. I’ve heard from a fair amount of people who are trying to sell their membership because they don’t use, or like, those campgrounds. Some here have mentioned “resale” as a less expensive way to go if you are interested in becoming a member.
You can get a lifetime Senior Pass if you are 62 or older for a one time payment of $10.00, $20.00 by mail, that will give you lifetime access and camping at National Parks for half price; including Grand Canyon, Yellowstone, Yosemite, etc.
US National Park PassesLots of State Parks also have reduced fare Senior Passes or reduced fair Disability Passes. Each State is different and some do require you be a legal resident of that State. I am a legal resident of California but Oregon gave me a reduced fare camping pass, and I did the whole thing, application, etc., online and then mailed it in. So some States will give you a pass even though you are not a resident of that State.
Other than that; as some have already mentioned, the monthly rate at most private campgrounds is usually cheaper than the daily or weekly rate.