So I'm not a geologist. But my friend is. So I can appreciate that this would be entirely exciting for some folks. We paid $2/person to get in. There was a sign that said "kids" under 18 are free, a perk for those with youngsters or teenagers.
I would concur with Carina in that the museum is fairly small, but houses a pretty impressive collection of rocks, crystals, forms, etc for it's size and scope. It's also kind of cool to see the amount of copper and unique geological forms that reside within Arizona.
We spent about 45 minutes or an hour here and I felt like that was plenty. The highlight for us was the Rose Mofford room, housing a ton of her awesome stuff. A turquoise encrusted violin (or viola...i'm not good with that stuff)...seriously? Pretty awesome. A ton of her plaques, photos, and interesting bits that she collected over her time are up on the walls. So that was pretty neat.
Also, they have a section of rocks that look like food. Geologists gotta get some laughs somewhere right!?