If you're wanting to buy one, I think the Campmor tarp Glenn mentions is a good mid-range choice. I've darn near clicked the button for getting one of those myself a few times.

I've made several tarps using the "$4 roll" of clear plastic Lori mentioned. My experience with them is that 2 mil plastic is just a bit too light to count on. 3.5 mil plastic works great, but it's a bit too heavy. It's still lighter than the blue poly tarps though.

My most recent experiment using plastic was to make a tent using SOL emergency blankets taped together. The plastic they use is about as tough, and lighter than the 3.5 mil clear plastic, so if I were to make another tarp from plastic I think I'd use the SOL blankets. And I think that would fit your requirements pretty good too.

It would cost about $20 to make one and that would include tape and a grommet kit for your stake holes. You can get a grommet kit at WalMart in the Camping dept near the blue poly tarps, or you can walk over to the Sewing dept and get one that has smaller, lighter, grommets.

Here's a video of a tarp somebody made: