This place takes total advantage of tourists. My dad and I rented mountain bikes here after reading about the trail to the Hoover Dam from Boulder City in a Rails to Trails magazine. This business was 'recommended' by the magazine. When we rented our bikes, my front tire was flat (they didn't bother to check them). They didn't provide us with a map of the area (which would have been helpful since the trail doesn't start at the bike shop. The rates also take advantage of tourists. The only way to avoid having to pay the full day rate would be to immediately turn around at the dam and ride back because the trail is a significant downhill from Boulder City to the dam.
If you have two vehicles available, i recommend parking one at the dam so you can bring yourselves and the bikes back since that will cost you an additional charge. All in all, they know that 1. you're not going to want to ride back and charge a fee for pick up, and 2. that if you take a tour you'll have to pay for the full day because it'll take long enough that your 3 hours will be eaten up