This is going to look like an unusual suggestion.

Sale #1: 1.2 buds

http://www.tf2outpost.com/trade/24204348

Sold with a Genuine Horace for a Cloud 9 Das Hazmattenhatten

2.2 * 0.9 = 1.98 round up to 2 buds

2 buds = 22.50 keys

22.50 - 9 = 13.50 keys

13.50 keys = 1.2 buds

Sale #2: 1 bud

http://www.tf2outpost.com/trade/24028785

Sold for 1 bud pure

Sale #3: 1 bud

http://www.tf2outpost.com/trade/23693170

Sold for 1 bud pure

Sale #4: Some huge outlier price

http://www.tf2outpost.com/trade/23569531

Sold for DP Lord Cockswain + 8 keys in sweets

Obvious outlier