If you let us know what tartan you are looking for we may be able to help find the best deal.

As Matt says it is economies of scale. The reason it is cheaper for the companies to do larger runs of any given tartan (typically the popular ones) is that, besides the cost of the actual yarns and the usage time on the looms, there is a rather expensive time element getting all the threads set up in the warp and weft portions of the looms (set up time) that is labor intensive. The more yards you can weave in one sitting the less per yard the set up costs are, so the more popular tartans, which get woven in large runs, generally cost less than those that require the same setup costs but get a lot less yardage made. It is not cost effective for the mills to keep a lot of stock of uncommon tartans on hand due to inventory and storage costs, with additional potential for loss should there be stock damage (moths, warehouse leaks or fires, etc...), so for the less popular tartans they typically only weave what they have ordered or maybe a few yards more so they don't have to set up the machine again when another stray order for that less popular tartan comes along. But they can weave hundreds of yards of the popular tartans as they know, after years of sales accounting, about how many yards per year they will need/sell of each of the more popular tartans, and plan that in their long term plans. Weaving in bulk brings down the costs.

If you want to see expensive try getting a few yards of a rare custom weave done----8 yards of single width fabric can cost you $80-100 per yard in some of them.