Proof below. Unusual overpay is applied.

5 months-old price.

1st sale:

http://www.tf2outpost.com/trade/11818340 sold for clean voodoo juju=18 keys at time of sale

2nd sale:

http://www.tf2outpost.com/trade/11108289 sold for 20 keys

3rd sale:

http://www.tf2outpost.com/trade/11821394 sold for blizzard dead cone here: http://www.tf2outpost.com/trade/11825563, =17 keys

4th sale:

http://www.tf2outpost.com/trade/13366593 sold here for 18 keys: http://www.tf2outpost.com/trade/13329016

5th sale (outlier):

http://www.tf2outpost.com/trade/13257544 willing to do 20-22, unable to track sale, probably sold for 14 keys? - http://www.tf2outpost.com/trade/13855387

6th sale:

http://www.tf2outpost.com/trade/13892731 bought smoking cosa with it along with blood banker and minor sweets: http://www.tf2outpost.com/trade/13866927, making it around 19 keys

7th sale:

http://www.tf2outpost.com/trade/12061826 sold/willing seller for 19 keys

8th sale:

http://www.tf2outpost.com/trade/14064751 sold for 17 keys

Sales: 14, 17, 17, 18, 18, 19, 19, 20

Mostly sales from around 17-20 keys, therefore the range.

This hat has dupes, especially the one I was tracking, it shouldn't matter too much as its pretty low-tier but it was difficult to track sales.