Rent a car from the airport or at a kiosk at a hotel and come out to the Dam! We didn't go on a tour but where else are you going to see something like this. Take a walk on the gorgeous Tillman Memorial Bridge and get some great snaps. Parking is free there.
Then drive down to the Dam. Pass the parking garage where it tells cars to turn in. You pay $7 if you park in that garage. If you pass it going down the road, you'll get to a free parking lot and you can walk by the Dam that way, too.
Also, the just visiting the visitor center cost $8. It may be worth it to do this and the tour, but we got there late. Last tour leaves at 3:45 PM and the Visitor Center closes at 5.
It's well worth a day trip. You probably don't need a fancy tour bus to take you there. Also, it's nice to get away from the strip for a few hours. Keep in mind, this is a tourist area too but not nearly as much foot traffic as the strip.