I think that AAC members get a discounted rate - was it 50%?
Certainly worth joining for those who will frequently camp there, and there are other benefits from joining.

While for sure there are some for whom $20 a night is prohibitive, a quick look at the vehicles in the Trapps parking lot or backpacks laid out at cliff base on any weekend makes me a little hard-pressed to feel even the full price is going to cause much financial hardship for the majority.