You would think that the glass shaker bases would break more than the metal lids, but I've seen a lot of lids that are nearly eaten through from salt corrosion. This Cameo green depression glass shaker lid is paper thin and only about half intact.
Yuck! You wouldn't leave the salt in your shakers would you?
Shakers are among the most reproduced depression glass pieces - think of the famous pink Cherry Blossom ones. There are only two pink sets reliably identified as authentic and they have been reproduced multiple times. Floral Poinsettia, Florentine #1 are patterns that have very few reproduced pieces, but the shakers are in there. Mayfair and Sharon have repro shakers along with other remade pieces.
I think people reproduced shakers because they were (and are) popular collectibles and real ones in good condition tend to be pricey. Not everyone really cares whether they have authentic glass as long as it's pretty and in good shape.
If you want to buy only authentic glass then I recommend any of the better depression glass guide books. Gene Florence and Barbara Mauzy authored excellent guides that describe the fakes and give tips to distinguish real from fake. A small investment in time can make you informed and a choosy collector.
Shopping Information: We have the salt and pepper shakers shown in our eBay store as a lot and separately in our larger store Cat Lady Kate's Elegant and Depression Glass.