How about a sliding scale based on one's ability to pay? National parks belong to all of us and no one should be prevented from experiencing them due to income level. 

We need to stop viewing nature as just another commodity, it's bleeping nature! We have already done far too much damage to our ecological system and the parks are the last refuges of the wild. This push to decrease national park funding is not just an attempt to shift cost to visitors, it is really just the set up to allow privatizing the people's treasure to the highest bidders (resource extractors). 

We need to remember https://www.youtube.com/watch?v=XaI5IRuS2aE