While not quite as popular or well-known, the legendary Topstone mask company sold a line of vacuformed plastic monster masks in addition to the rubber masks so often seen in monster magazines. Many of them were cheaper (no pun intended) knock-offs of the same designs made famous by Topstone's top masketeer, Keith Ward.
One excellent example of these vacuformed Topstone's was a 2011 eBay auction of the Vampire Girl mask. Offered in apparent like-new condition, it was up for bid at a price of $499.99. I came across this after the auction had ended so I don't know if it was sold or not.
For the sake of historical preservation and Monsterologists everywhere who might have missed it, I have included the complete series of images that accompanied the listing.