Proof below. 3 month-old price.

Last priced this as my 1st unusual suggestion (my format has changed a lot). Now it's my 100th. How fitting xD

Sales:

http://www.tf2outpost.com/trade/15939160

-wanted 18 keys

-seems to have sold for 18 keys (bp histories unclear)

http://www.tf2outpost.com/trade/15032633

-was looking for 21 keys

-sold for steaming milkman=17 keys

http://www.tf2outpost.com/trade/14963792

-steaming cosa+10 keys+#60 crate bought smoking noir=7 keys, clear outlier

http://www.tf2outpost.com/trade/15058734

-b/o of 22 keys

-sold for steaming beret+3 keys=16 keys in a day

http://www.tf2outpost.com/trade/14910818

-cosa+g. planeswalker seems to have sold for stormy mullet: http://www.tf2outpost.com/trade/14909990

-18 keys

http://www.tf2outpost.com/trade/14670710

-b/o of 20 keys

-sold for 20 keys

http://www.tf2outpost.com/trade/14430291

-b/o of 22 keys

-sold/bought for steaming stovepipe=19 keys

-stovepipe: http://www.tf2outpost.com/trade/14668895

http://www.tf2outpost.com/trade/15291446

-valued it at 17 keys

-sold for big kill+s. pan+3 keys=18 keys

http://www.tf2outpost.com/trade/15263332

-wanted 18 keys

-probably sold for the 18 keys even though only 17 keys can be seen between the 3 day-old gap of bp histories, he was buying the last key here: http://www.tf2outpost.com/trade/15226982

-buyer's bp:

http://backpack.tf/id/76561198035244571?time=1382684400

http://backpack.tf/id/76561198035244571?time=1382943600

Current trades:

http://www.tf2outpost.com/trade/15496540

-wants ~26 keys, 1 month

-notable offers:

planets brigade+listed sweets=18

o.f. noir+1 key=17

http://www.tf2outpost.com/trade/15888417

-wants 24 keys, 2 weeks

-no notable offers

http://www.tf2outpost.com/trade/15437705

-notable offers:

1 bud+2 keys? (not sure if for cosa)=18 then

-offers from here: http://www.tf2outpost.com/trade/14784650 (was open for a month)

18 keys

1 bud=19 then

1 bud+2 keys=19 then

19 keys (retracted)

1 bud+3 keys=20 then

http://www.tf2outpost.com/trade/15989234

-supposed c/o of 20 keys (no proof though)

http://www.tf2outpost.com/trade/15505097

-notable offers:

1 bud=16 then

16 keys

1 bud=19

Sales: 7, 16, 17, 18, 18, 18, 18, 19, 20

Prices in keys since there are sales below a bud. I believe 16 may be too low since that sale was only 1 of 2 offers posted on the trade in the 1 day that it sold in, so probably low as he could have gotten more if he had left it up for longer. So not including 16 in range. Other than that the range is pretty straightforward. Unusual overpay and original prices used.