This is a predictable seasonal subject for my blog: super low one-way rates in the spring out of Arizona, and the reverse later in the year back to Arizona.
Hertz has bargain rates of $5 per day for economy or compact cars driven one way from anywhere in Arizona to any Hertz location in California (or Utah, Colorado, New Mexico, Texas, Oklahoma, or Kansas).
For just a little more per day, you can rent an intermediate, standard, or full-size car, or a mid-size SUV. Keep in mind that the only additional mandatory cost is tax; there isn't any one-way drop-off fee. And if you rent from a non-airport location in Arizona, you will avoid airport concession fees.
Pick up the vehicle any day starting on 4 April but no later than 30 June. You may rent the car for up to 14 days.
Here are three examples. Prices include tax.
15 April to 19 April - Phoenix airport to San Francisco airport
economy car: $60.54, mid-size SUV: $139.96
21 May to 23 May - Tucson off-airport location to LAX
economy car: $16.58, mid-size SUV: $50.79
11 June - 20 June - Flagstaff Amtrak station to Chico
economy car: $57.44, mid-size SUV: $203.70
For complete details see this page on the Hertz website.
Avis has a similar but not as low-priced offer, and other rental companies may as well.