wrt the above suggestion to fly the cheapest one-way PHX-LAS: see what the cost difference is to pick up the car at an off-airport location in PHX and return it to the airport (yes there will be taxi fare to the rental car place, unless it's at or near a hotel that has airport shuttle service ... although if you're arriving late in the day the trip to the hotel is part of the itinerary anyway)

this avoids ridiculous surcharges that local jurisdictions and authorities impose on airport concessionaires -- mainly to prey on the traveling population, both business and leisure