I just made mine the other day. Two sheets of 1/2" foam insulation (4'x8') from the home improvement store ($9.51 each). I cut 1 foot off the long ends because 1) we rent and I cant attach them to a wall and need them mobile without fighting against the ceiling height, 2) I dont have a blank wall large enough and have to "lean" them against the closet door/wall or my shelves. I bought flannel from JA's. Sewed a seam down the center and then covered them in that. After using my coupon at JA's I think total investment is approx $25.
I considered a sheet and the curtain methods, however sometimes my blocks are a little large and therefore heavy and if I have paper pieced a block and have not removed the paper yet, it will not stick. I like being able to just "stab" it instead of the in/out method of pinning. But a 1/2" inch is more than sufficient
My boards were not pink, just silver on one side and grey with blue writing on the other side. Also they had these bundles of Styrofoam that were 32sqft total coverage but in smaller pieces. I think a bundle was $10.50