Let's just begin.

Proof:

#1

Sold for B.M.O.C. (9 keys) and 32 keys pure = 41 : 12.5 = 3.28 ~ 3.3 buds, although you can get almost 4 buds with 41 keys these days

http://www.tf2outpost.com/trade/24217112

#2

Vintage soldier's stash painted the color of a gentlemann's business pants (~1.5 ref) all together 1.3 keys, unusual Hetman's Headpiece (circling hearts) (~1.5 buds), unusual Madame Dixie (circling tf logo & aussie paint) (~1.2 buds) and a pro killstreak big earner kit, but since there is no price to them, im gonna be a devil and use a community market price, which is about 2.5 keys. All together they are 3.7 keys + 2.7 buds = ~3 buds

http://imgur.com/Cu9aSbz

http://imgur.com/OZpLFYF

#3

Sold for 2.7 pure

http://imgur.com/o6wpgyB

#4 (Xtra)

Sold it for 35 keys pure: http://puu.sh/dCkwi/031408a832.png, ~2.6 buds, these days ~3

If you find problems in my suggestion, please tell them in the comments and I try to fix them, also if you find more proof go ahead and post it in the comments.

Thanks for your time and have a nice day :)