Unusuals are priced at average ((Low End + High End) / 2)

Verdict:

High End - Seems to be able to go up to at least 20 keys, with outliers going as high as 26 in mixed, 23 in pure

Low End - Easily gets offers of 17 Keys, 18 too seems easy

Proof:

http://www.tf2outpost.com/trade/10239911 - 16 Keys + 1 Ref, 17 Keys, 17.5, 16.5, 1 bud, 27 Keys

16 Keys + 1 Ref

N&B Hermes w/ 3 Key (at the time of offer) Halloween Spell (17 Keys)

N&B Conquistador (15) + 2 Keys = 17.5 Keys

Steaming Safe 'n' Sound (16.5)

1 Bud (likely an outlier)

Smoking Counterfeit Billy Cock (21 keys) + Strange Sapper (6 Keys) (also likely an outlier)

http://www.tf2outpost.com/trade/9774957 - 16 Keys, 20 Keys, 16 + Sweets x 2, 19 Keys, 20 Keys, 18 Keys, 20 Keys - Sweets

Steaming Capotain (19.5)

N&B Bonnet (13) + 3 Keys

Blizzardy Dixie (20)

Bubbling Noir (16) + Sweets

N&B Armored Authority (16) + Sweets

Offer to Pay B/O of 19 Keys (the b/o has since been lowered)

Buds - he adds 3 keys (buds were 23 when offered)

15 Keys + 3 Keys in items

20 Keys - Seller offered to add some sweets

http://www.tf2outpost.com/trade/11096404 - 19

19 Keys, offer same day as posted

http://imgur.com/ZSQiwmf.jpg

Bought for 20 and sold for 22