Plastic you buy from Home Centers is cheap (affordable) BUT:
I replaced my pvc/rebar hoophouse cover 4(?) times one season, due to
installation issues (ie, not tight enough). I think I was using 6 mil
plastic. It tears out pretty quickly where the plastic has been
folded.

Where to get the good stuff:
Northern Greenhouse Sales
http://www.northerngreenhouse.com/index.htm

Great products, good people. My covering from them cost more than the
cheapo stuff, but it is the heavy duty woven plastic and my (now loose)
cover has lasted 5? seasons with periodic winds 40-80 mph. Like I
said, good stuff.