My friend who owns a ranch tells me the price breaks start at the 1,000 gallon point on gas. I don't know what it'd be for diesel.
He claims he saves about 30% buying 1,000 galons at a time. I do know that some of the big ranchers send trucks directly to the gas refineries here in Houston and haul it themselves, and they supposedly really save a bundle. The price difference between the refinery loading dock and your local gas station is about double, plus transporation costs, so if gas is selling for about 1.80 at the 7-11 here in Houston, its going for about 90 cents at the refinery. Thats not always true, but its a pretty good rule of thumb.