Is this a good price for tyvek? $1.55 for 1×9

I paid $2 a foot from anti-gravity gear (had some other bits to buy as well) so I'd say that's a decent price- not sure what it costs from a home center type store though?

If you have any construction going on around you, you should be able to find some for free. It may require some "dumpster diving". If you buy it from a building supply store, its pretty expensive for a roll (a couple hundred dollars).

The best price depends on how much they charge for shipping and how much you are buying. Here is the best deal that I found for the quantity that I ordered:

http://www.ebay.com/itm/TYVEK-ultralite-9-ground-cloth-sheet-camping-tarp-tent-footprint-/200655651648?pt=LH_DefaultDomain_0&hash=item2eb8024340

If that link doesn't work, follow this one:
http://myworld.ebay.com/hike_n_light/
and look for "TYVEK-ultralite 9' ground cloth sheet/camping tarp/tent footprint"