Have you ever considered making your own.
I damaged the Pertex fabric on one of my favourite jackets, and very high quality Ripstop Nylon is available from companies on Ebay, for as little as £3.99 a meter squared, and available in quite a few colours.
Buying a already manufactured tarp, is bound to be expensive, but purchasing the material yourself is a very low cost option.
The Pertex Rip stop fabric i received through the post on Monday, is very high quality stuff, as heavy duty as Pertex 6 i recon, maybe harder wearing as well.
Buy the material yourself, and experiment a little, as i recon you could make your own for less than £15, heavy duty Ripstop nylon is incredibly cheap to buy online.