Not too bad of a drive from Vegas. Thankfully, we were already renting a car and didn't need to pay for a tour. What we wound up doing was parking on the Arizona side of the access road, after the paid lot there are some free parking lots. We then walked down the hill and checked out the dam from the road. I imagine the walk back up the hill and climbing the stairway would get tougher in summer, though.
So, seeing the dam from here was ok, but what you REALLY should do is check out the dam from the Tillman Bridge walkway. There is a parking lot off of 93, you then either climb (what would be a few floors in an office building worth) of stairs or take a series of ramps to access the walkway that runs along the side of the new US 93 bypass. From here, you get a fantastic view over the whole thing for free.
So if the first part of our dam trip was three stars and the second five stars, I'll say four stars overall.