Mini for Professional Killstreak Strange Festive Sniper Rifle:
Week on Steam Market:
53.30
55.00
46.00
55.12
61.61
54.99
59.16
65.00
61.00

9 Sales = 511,18 / 9 = 56,7 (Average Price) / 2.45 (Current Key Price) = 23.1
23.1 * 0.85 (Taxes) = 19.6 + 23.1 / 2 = 21.3
21 Seems good, since it took me about a week to sell one that I had at 20 keys:
http://www.tf2outpost.com/trade/25792467