On exploring some of our national park entrance fees, I've come across several that will be increasing their fees this year.
Yellowstone & Grand Tetons 7 day car pass goes from $20 to $25 as of May 1, 2006 http://www.nps.gov/yell/planvisit/orientation/entrfees.htm
Grand Canyon 7 day vehicle pass goes from $20 to $25 on May 1, 2006. http://www.nps.gov/grca/pphtml/newsdetail22261.html
Death Valley's fees increased as of Jan. 1 http://www.nps.gov/deva/Pdf/NR_Fee_Increase.pdf
Bandelier National Monument http://www.nps.gov/band/pphtml/newsdetail21551.html
Joshua Trees National Park http://www.nps.gov/jotr/pphtml/newsdetail21817.html
These are just the ones I've found. There could be more.
The National Park pass is staying at $50 so it will be a much better value!