Was in my local Tesco's the other day (Bedworth in Warwickshire) and my son wanted a mask. I looked at them and they were plastic ones and I was going to get him one, until I noticed they were 14p (may have been 13p in fact) each. Bought one of each (there were 3 different types).
Not great quality, but brilliant for kids to play with around Halloween. A picture is shown of one of the masks.
No idea if this has been posted before or not, I did do a search, but the search function here is not good to me.