I almost always get the best rates through
Costco Travel. Since they don't require any money until the time of rental, I usually search for and book a car months in advance, then search again on a weekly basis, rebooking and canceling my original itinerary if I find a better rate later. Right now I'm seeing rates on Costco Travel starting at $651 with Alamo SLC-PHX, although I just guessed at a 2 week timeframe in late April to arrive at that quote. Rentals through Costco travel typically also have unlimited mileage and an additional driver fee waived, although spouses don't usually incur this upcharge anyway.
As honeybbq said, this is probably a higher rate because it's a one-way rental. The rental companies have higher overhead on these due to either trying to line up additional one-way rentals on the same car, or in some cases even having to title transfer the car across state lines.
Also, as mentioned by Mother Fussbudget, I always decline all the up-sells at the counter (insurance, fuel, GPS, etc). If my rental is relatively low risk I just use my own insurance and book with one of my credit cards that offers decent secondary insurance (Chase Freedom Visa Signature has decent secondary), and if I feel like it's a higher risk rental due to location, duration, weather, etc then I'll book the rental on a credit card that has flat-rate primary insurance (in my case I have this added to my American Express).