For one-way rentals they include a certain amount of mileage and days in the rental and charge $0.40/mile for every mile over and $40/day for every day over. Then of course there's the extra gas, and those trucks even empty don't get very good gas mileage.

I did some quick calculations on Santa Monica to Austin, and the numbers ended up being a wash assuming a 1400 mile trip and 8 mile/gallon mileage @ $3/gallon. Not accounting for the extra hassle and risk of getting into an accident.

For trailers this can work, because they don't charge mileage on those. I know someone who did an in-town rental and drove it to another state to pick up some stuff because it was cheaper that way.