I catch such few big fish that I never lose my plugs so I don't mind paying an arm and a leg. As long as I tie good knots and don't cast off the plug, I can keep the same plug on all year.
I don't think its necessarily supply and demand setting the price on the secondary market. Its almost like a bubble where people see what plugs go for and then thats now the price. I've seen brand new plugs go for $x online when their $10 cheaper in B&Ts right in the same area the person is buying them is from. I'd rather have the builder make the $ rather than someone who is taking advantage of someone else b/c they know where to find the plugs.