As said before, supply and demand, and supply on them is LOW.

Fires are going for CHEAP these days, and pastels even cheaper. You could likely get a pair for under $300 (probably low quality as far as color goes). HOWEVER, fireflies are still fetching over $1k a piece, simply because there just aren't that many of them despite the quantities of fires and pastels that were produced last year.

Now regarding fire spiders, I haven't even seen one of those for sale lately, and I don't think i've ever seen one in person at any of the shows I've hit up.