no proof for the low end?

You'll need to apply overpay, and find two sales. And where does 4 come from?

http://forums.backpack.tf/index.php?/topic/6033-new-unusual-price-suggestion-guide-the-cheat-sheet/'

"All OFFERED hats and items should be taken at 90% of their original average price."

You need to apply overpay.

Two sales are required to establish a range on unpriced hats. If the two sales are discrepant, a third sale is required to determine the outlier. Otherwise, read Exception 2

Also, can't get a range from 2 sales. Needs some work, but good start :D

As indicated, if there have been more than 1 sale in the last 2 months, a hat can't be priced based on a single sale.