Why have x10 devices gotten so expensive?

I was looking through ebay the other night and noticed that the cost of x10 modules have doubled and tripled in price. I was buying wall switches for about $6.00 and now they are $15-$20. Plugin filters were $4.00 and now they sell fo r almost $20 a piece. The cm15a is selling for over $100. I bought mine for $39.00 last year. Glad I have a few spare cm15 and cm19 modules. I thought about selling them to reduce clutter but now I might just hang on to them. At these prices it seems more economical to buy zwave devices instead.