For primary fermentation plastic is fine, but for secondary glass is preferred for a number of reasons. At a home brewer's shop they'll sell you a 7.9gal plastic bucket for like, $20. I made one exactly the same (though the bucket wasn't the nice big 7.9gallon thing) for $1.35. Here's what I did:
I asked at Giant Eagle's bakery and they gave me 1x 3gal and 1x 4gal plastic bucket. Then I found what size Rubber Grommet is needed to accommodate an airlock, here is the size:
Outer Width: 5/8"
Inner Width: 3/8"
I drilled a 3/8" hole in the top of the bucket because that was the biggest drill bit I could find at the time. That was not big enough so I had to widen it. I suspect the right size hole is either 1/2" or 5/8". My hole was a little sloppy because I didn't have the right size drill bit, but it is definitely very airtight, and it is exactly like the bucket you can buy for like $20 (bucket, lid) at a winemaker's shop.
All in all, it cost me:
Rubber Grommet at Do-It Best: 35¢
Airlock at the local Winemaker's shop (you can get it online too): 90¢
Bucket at Giant Eagle's Bakery: Free
Total = $1.25 / bucket