z+4

Selling/Sold

http://www.tf2outpost.com/trade/12533628 - Selling for 1 key + 3 ref (2 added posts)

http://www.tf2outpost.com/trade/12808648 - Yellow for 1 key + 1.66

http://www.tf2outpost.com/trade/12553130 - 2 (Aqua + Yellow) for 3

http://www.tf2outpost.com/trade/12541302 - Bought 5 buds for 16 random keys each

http://www.tf2outpost.com/trade/12785624 - 2:3, sold none so far.

http://www.tf2outpost.com/trade/12812126 - Yellow for 1 key + 1.33

http://www.tf2outpost.com/trade/12788609 - 1 key + 2 ref each (Sold one aqua, one brown and and unknown amount of keys of unknown colors to 2 other people.)

http://www.tf2outpost.com/trade/12812013 - Selling Yellow for 1.5 keys each

http://www.tf2outpost.com/trade/12792628 - Yellow, Orange and Green 8 ref each (Sold Green and 2 Yellow keys)

http://www.tf2outpost.com/trade/12770360 - 1 key + 2 ref each (Sold Red, Brown and Yellow.)

http://www.tf2outpost.com/trade/12798764 - Yellow for 1 key and 2 ref

http://www.tf2outpost.com/trade/12591665 - 1:2, 2:3 (and several other equivalent amounts, sold a few apparently.)

http://www.tf2outpost.com/trade/12715663 - Selling 1 key + 3 ref each (Out of stock: Red, Aqua, Yellow, Green and Brown.)

http://www.tf2outpost.com/trade/12800725 - All colors, 1 key and 2 ref each

http://www.tf2outpost.com/trade/12606444 - Sold many of all colors at various prices. (Yellow ~6.44)

Buying

http://www.tf2outpost.com/trade/12607118 - Bought 9 at 1 key + 0.33 each

http://www.tf2outpost.com/trade/12600231 - Buying 1 key + 0.33 each, bought 1 (orange?)

http://www.tf2outpost.com/trade/12777277 - Sold buds for 20 random keys (buds being at 23 keys, each is ~1.2)

http://www.tf2outpost.com/trade/12534907 - Paying key + 0.33 each, 3 added posts (1 red)

Steam Community Market

Yellow - http://steamcommunity.com/market/listings/440/Yellow%20Summer%202013%20Cooler%20Key

Median sales of all color keys is $2.20 (~1.2 keys)

Note: Trades with ambiguous colors of keys are included in all of the suggestions. Buyers @ 1 key have got nothing (thus are not included). 1.1 range is supported by the buyers at around key and 0.66 and the community market (keys being at ~$1.80). People asking for 1.7+ keys can't get their keys sold either, so that's off the range.