I would say National Park trips are "expensive" in ways other than the cash it takes to get in. We tried to plan our trips avoiding the major holidays and spring/fall breaks of the local school districts, etc.
- They're overly crowded. It feels like a theme park. Looking at a waterfall and you're standing shoulder to shoulder with strangers. It's crazy.
- Parking is nearly impossible to find. Some days you have to pay for parking and bus into the park.
- Every campground was full. Hotels and BNBs are not exactly cheap in those areas.
We'd ask workers/etc about the traffic and they'd always say stuff like, "oh yeah, this is nothing. You should've seen it X weeks ago!"
So, sure the National Park itself is peanuts to get admission. But it was disappointing in a lot of ways. For those reasons, I would be more interested in paying more dollars to see State parks, if all the other "costs" were "cheaper" :)
Everywhere you could look you could see damage to the park. From people swimming in the mountain lakes, going off trail, cars providing pollution and noise and killing animals, etc. The volunteers at the park were exhausted. And I'm pretty sure someone hit and killed a black bear with a car on the weekend we were there. People were just moving through the park like cattle at the mall.
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).