Check Rock Stone and Sand Yard in Lorton too.
http://rssy.com/
I moved about 20 tons of rock and gravel a few years back and they were the cheapest around by quite a bit. At least assuming that you have a good truck and can pick stuff up yourself. If you're having it delivered then that will change the cost completely depending on where, what, and distance.
And if you are picking up yourself then unless it's just a trivial amount, you will need a *good* truck. Don't even try it in a car, SUV, etc. A small amount of rock/gravel weighs a ton (literally) and doesn't go far when you start laying it down on a sq. ft. basis N" deep, layering it when building walls, etc. Even with an F250 or similar, you can overload it with what looks like not much gravel/sand. 1/2 pallet of field or flagstone will be about it to do easily. Depending on what it is a full pallet is somewhere around 1.5 - 2.5 tons and at the high end that's pushing your luck it on public roads for any distance. While you may still be within load limits, that's not counting having idiots stopping short in front of you, etc.