I had a similar situation, pick up at PHX with a drop off at LAS. You generally have either a one way fee or higher rate. The exception is when the computer determines either cars are needed at your destination, maybe a big convention, or a specific car needs to be moved. Keep looking try any vehicle class that works for you. At one time one of the online TAs, I think it was travelocity, had better one way rates then others. I'd also try autoslash.
FWIW
Alamo didn't let me pick my own car. I was assigned a car with Nevada plates. The reeked of cigarette smoke. I asked for a different vehicle. I had to wait for them to see which vehicle they wanted to give me. It was another car with Nevada plates.
I kept looking. Price kept going up then was not available at all. One way rentals don't follow the general rule. They'll only go down if the agency wants the car moved. Wait too long and there is a chance it will be sold out, meaning the agency doesn't want the car moved at any price, renters are willing to pay.
Two weeks...occasionally you'll come out better if you switch cars halfway through. Will you be near PHX at the half way point?