Polycryo is both inexpensive and very light, not to mention it's easy to build with. The only problem is durability. Folks say if it starts a run, it will keep going, though I haven't experienced that yet. If you do use it, make sure you "hem" all the edges with that 1/4" double sided tape and that all the guy lines have elastic segments.

If I had the time to make another tarp, I'd probably try silpoly myself.