It's easy to lost perspective when referring to stops due to the fact that it is a logarithmic scale. When talking about ND filters each stop represents a halving of the intensity of incoming light. Like grains of rice on a chessboard it starts to add up very quickly.

A 15 stop ND filter reduces incoming light intensity by a factor of 32,768. That's enough to turn what would be a hand-holdable 1/50s exposure into well over ten minutes. This makes it a pretty specialist piece of equipment.

Given the specialist nature of a 15 stop filter (which is therefore manufactured in small quantities, unable to benefit from economies of scale), along with difficulties in actually producing such a dense filter that doesn't result in a strong colour cast it is unsurprising to find one with a $450 pricetag.

If you really need to be able to do ten minute exposures in broad daylight, then a more economical solution would be to pick up a 10-stop ND filter shoot 30 exposures of 20 second each with an intervalometer, and then stack them in software. As a bonus you'll also get a boost in dynamic range!