I was camp host last year. There was no fee involved. There is a consession campground on the same road with water and electric. Could be the one you saw.
Checked website for Spring Creek Campground, West Yellowstone, Mt ,. Did not show fee involved.
There is a commercial campground in Montana called Spring Creek. Might be the one you saw.
The one I am referring to is 18 from west gate.
Hope this helps.