Proof:

Buyer:

http://www.tf2outpost.com/trade/8380271 - Buying for 1.5 keys

Sellers:

http://www.tf2outpost.com/trade/14518223 - Sold in few hours for 1.2

http://www.tf2outpost.com/trade/14523264 - Sold in few hours for 1.2

http://www.tf2outpost.com/trade/14518733 - Sold in few hours for 1.2 (somewhere in this page it's listed as 1.2) http://www.trade.tf/listings/search/Sell/Strange/Widowmaker/all/5

http://www.tf2outpost.com/trade/14542309 - Sold in MINUTES for 1.2

http://www.tf2outpost.com/trade/14521874 - Sold in hours for ~1.32

http://tf2tp.com/trade.php?trade=9790631 - Sold for 1.33

http://www.tf2outpost.com/trade/14541024 - Sold in less than 2 hours for 1.33

http://www.tf2outpost.com/trade/13428308 - Has offers of 1.33 and 1.5

http://www.tf2outpost.com/trade/14308848 - Sold at least 1 for 1.5 to: http://www.tf2outpost.com/backpack/76561198061688224

http://www.tf2outpost.com/trade/14280681 - Sold with Strange Part: Heavies Killed attached (1 - 1.33 ref) for 2 keys, which places Widowmaker at ALMOST 2 keys

http://www.tf2outpost.com/trade/13536094 - Declined an offer of 2 (by [SOUPRIZE] DeeblowAce) I have no idea why he did that...

I know not MUCH proof of 1.5 but 1.2 sells EXTREMELY fast, some sell for higher, some are in sets/overpriced/strange parts/way too old to know if and when sold and such, and that's all out of less than 6 pages on op

At the moment of posting Classifieds had 4/4 for 1.5