You can always go for a lifetime membership to Good Sam...maybe...just maybe...the 10% discount will offset the cost of it in your lifetime.
As mentioned, parks pay to be listed. They also get rated and give customers a break on the price. Passport America parks are wanting to fill spots in their parks. Most of the time its when their attendance is low (during the week). The parks give a 50% discount to fill those slots. I guess they figure half the money is better than no money. I'm not sure, but I don't parks pay Passport America to be listed. And they don't have to worry about being rated either.