I frequently get weekend rentals in Portland and they are very cheap. I generally rent from budget downtown so I don't get hit with any airport taxes, but my total bill is rarely more than $13 per day (that's for a small car, of course, but about half the time, I end up with a free upgrade).

Budget doesn't charge drop fees either, nor do they harrass about things like insurance and pre-paid gas (helps to have fastbreak).