The U.S. Geological Survey offers a Lifetime National Parks Pass for Senior Citizens (age 62 or above) for $10. To get this price, you'll need to purchase it in person at one of these locations. The price of this pass is set to increase to $80 sometime this year, as reported by the AARP. It grants access to all 417 national parks and over 2,000 other recreation areas.
Alternatively, you can buy one online for $20 plus $5 for shipping.