During our travels through the US they definitely provided the most bang for the buck, especially given that we had the $80 annual pass. In that context it was actually state parks that were an additional cost for us, though certainly never any kind of substantial cost. The tiniest fraction of our budget. Doing stuff in cities was much more expensive and we didn’t even consider visiting Disneyland, even though we actually stayed a couple of nights in Orlando (to visit the Kennedy Space Center – one of the most expensive things we did - followed by a visit to the neighboring state park, which was very cheap).
We never stayed inside or even just near the parks, so that certainly helped. Our budget of spending between $100 and $150 per night for three people always felt very manageable (mostly AirBnB and so we nearly always had a kitchen to cook food) but I can see how that might be too much for some people.
I think had we wanted to stay closer to the national parks (to explore them more thoroughly instead of basically doing day trips to like twenty of them) we would have spent a bit more on places to sleep (more like $200).
The fun things to do in national parks don’t cost any additional money, at least usually. I guess if you want to ride a mule down to the Colorado River in the Grand Canyon? But even that’s more limited by scarcity than price. Maybe there are little costs here and there (like renting bikes in the Everglades) but we definitely spent more on t-shirts about national parks than we did on things to do in national parks. They felt like a real benevolent gift most of the time, expect when there were too many other people, I guess (though since we were one of those too many people we could hardly complain).