Loc: Kitsap Peninsula, WA
I usually use a thin plastic painter's tarp for a ground cloth under my tarp. It has always worked fine, but I think I could save a little weight by simply using one of those Space Blankets I see advertised on Ebay for a buck. I was wondering if anyone has done that. I know they wouldn't last long. The plastic I use has lots of duck tape patches and isn't vary durable either.
I used an e-blanket, though it was far more than a dollar, as a ground cloth on some desert backpacks. It served a second purpose as a sun shield, as my wife doesn't fare well with heat at times. It acquired lots of rips quickly. I've had better luck with crossed linked polyethylene , sold as polycryo, or patio door insulation. You need the 1.5 mil, not the .75. This is pretty durable and a few ounces for a solo groundsheet. I'm guessing the dollar stuff is meant for one use and would be in the trash very quickly.
My buddy uses one as a ground cloth under his tent. It is difficult to work with because it is so light and flimsy. He usually shreds his setting it up and it never lasts more than one set-up. Its always seemed like a really waste to me.
Loc: Eastern MA, USA
This may be another YMMV situation. I use a mylar blanket slung under my hammock. Yes, it is noisy, but one can last me all season and beyond. Also, I just returned from a trip to FL. On my bucket list was camping at a waterfront site in Fort DeSoto (County) Park. I used a space blanket under my <$20 (well-known bidding site) tent. The base was mostly sand with some small shells. Even with moving the tent, between sites, the space blanket looked pristine when I put it away. No, it did not fold as small, but it at least was the same weight.
That said, if you have the opportunity to use Polycro, my opinion is "Go for it," unless you are in a sun-baked situation. I also used a one of these blanket under my tent in the Grand Canyon some years ago. With afternoon temps reaching 120 F, the ground didn't cool much before about 1 AM. The first night, the heat was somewhat reflected away from me, the second night (not moving the tent) the ground hadn't heated under the tent.
In windy conditions, you may find it easier to spread the lightweight blanket after erecting the tent, then working the blanket under it.