Found the answer to my own question! Here's the information - http://www.nps.gov/yose/planyourvisit/feesandreservations.htm For a personal car, it's $20, which is good for a week. The fees are a little different if you are a commercial vehicle, or if you are entering the park on a bike.

If you're planning to visit multiple parks and federal lands over the course of the next 12 months (or any 12 month period) you might consider purchasing a nationwide pass called "America The Beautiful Pass." It's really a great deal! Visit www.parkpass.net to get one.