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" :)