Biggie and Johnn both valid points. I've purchased names for $xxxx and hand registered names as well.
My thought though was that as a domainer, usually (hopefully) will carve out some sort of niche.
Maybe it's a hand reg domainer flipping available names, or purchasing higher quality names which likely command a higher price.
I assume that over a bunch of purchases it will likely skew one way or another.
But maybe I'm wrong on this one. Obviously each domain is different and no one says I'm willing to spend X for each domain they want to buy.