A couple guys mentioned this, but I'm not sure how clear they where. When you buy gravel in bulk, meaning it's being delivered in the largest trailer possible, is when you get the best price. Depending on who you get it from, the trailer will either be an end dump, or belly dump. End dump trailers never spread anything. Belly dump will, but it's still going to need spreading by the tractor or something with a blade. In my experience, this is always sold by the ton. The driver comes straight from the quarry and you pay based on the weight of what was loaded onto the trailer at the quarry. Usually 25 to 27 tones per load is normal.
When buying in smaller quantities, it's sold by the yard. It's already been hauled from the quarry to a yard somewhere, and they load the dump truck at the yard with a front end loader of some kind. The ones I've seen have all be one yard buckets, which makes the math easy. Buying it this way is about twice as much compared to having it hauled straight from the quarry. Having it spread is pretty common when having the smaller dump trucks deliver it.
What type of truck is delivering the gravel? 20 tones isn't very much. How long is the driveway? 4 inches is the minimum thickness for road base gravel to lock together and shed water. Thicker is even better. I plan on getting 100 feet for every 25-27 ton load.