I would suggest checking out FOX rent a car - they don't always come up on all third party searches. I did this exact same thing 3 years ago, just in reverse (Flew DCA-LAS, one-way rental, driving from Las Vegas to Phoenix, and then flew PHX-DCA).

I was getting crazy prices also for the one-way rental, but fox didn't charge me a drop fee for the one-way, was hundreds less (don't remember exactly...all the companies were roughly the same, like $45 a day, but fox didn't add the $400+ drop fee).

Anyhow, worth a look.