This guy: http://stores.ebay.com/Richards-Fabrics-and-Trimmings usually has some tartan for sale. I usually see PV selections from him, but he only charges ₤5.25, which works out to about $10.50... which doesn't seem like a bad price to me.

I once had a gent charge me ₤15.99 shipping for 6 meters of 11oz Dress Stewart. Very nice material... and I probably couldn't find 6 meters of the same material elsewhere for $165 total. The gent in question can be found at: http://stores.ebay.com/Tartanmclan44