Under $20 for all in at an airport is going to be tough.
Having said that, obviously supply and demand will come into play.

However, I have had good luck at SMF with all in prices ranging between $21 to $25. Usually, booked within a week's time or less.

I recently had three rentals out of SNA, all for under $20, but it was right after Thanksgiving Holiday, so perhaps demand was slow. Additionally, SNA appeared to have lower aiport taxes added on top of the base rental rate. For instance, my base rate was $12, and my total after taxes was $17.

One thing to consider is weekend rentals are almost always cheaper at most airports-very similar to hotel prices, as weekend rates are geared toward leisure travel rather than business.