The admission is only a $2 parking fee ($1 on weekdays), and it provides access to some wonderful petroglyphs scattered on lava rocks towering above the valley. Off to the west you can also see the cones of the ancient volcanoes that originally spread the rocks here, which accessible via unpaved road.
There are three sites scattered around the monument, the best and most popular being Boca Negra Canyon. You'll be able to see the most petroglyphs here. Get a map/guide at the visitor's center on Unser Boulevard, or print one out online before going.

