And the correctt answer is:

Go on ebay and find the outfit or two that are selling tyvek tarps WITH Tyvek tie outs already put on. That will set you back < $25. The Tyvek will be tougher for use on the ground, should you want it for that purpose, than every mentioned alternative that doesn't weigh 4 times as much. The Tyvek is also more resistant to pinholes from campfire cinders, too. Cooler under the sun. And recyclable into other things if you move on.
Did I mention it doesn't stretch much when wet?