Proof is all from the past month. Sniper Rifles with high-value parts or killstreaks have been excluded unless they fall within or below the suggested price.

Sold:

http://www.tf2outpost.com/trade/24089395 42 keys (proof: http://imgur.com/GwYi5uQ ) - screenshot credit to Jordan

http://www.tf2outpost.com/trade/24028306 43 keys

http://www.tf2outpost.com/trade/24364901 43 keys (in ~2 hours)

http://www.tf2outpost.com/trade/24082147 44 keys

http://www.tf2outpost.com/trade/24214406 44 keys

http://www.tf2outpost.com/trade/24231824 44 keys

http://www.tf2outpost.com/trade/24203796 44 keys

http://www.tf2outpost.com/trade/24082147 44 keys (Bill's Hat + 41 keys; screenshot: http://i.imgur.com/sUMsUyi.png ) - screenshot credit to Carnage

http://i.imgur.com/DdnDFsr.png . . . . . . . . 44 keys - screenshot credit to Carnage

http://www.tf2outpost.com/trade/23997703 45 keys

Credit to Carnage and Jordan for some of the proof.

Summary of sales:

1 x 42 keys

2 x 43 keys

6 x 44 keys

1 x 45 keys

Sellers:

http://www.tf2outpost.com/trade/24361259 43 keys (young)

http://www.tf2outpost.com/trade/24366618 45 keys (young)

http://www.tf2outpost.com/trade/24352169 45 keys

http://www.tf2outpost.com/trade/24349870 48 keys (specialized with a few parts)

http://www.tf2outpost.com/trade/24337949 55 keys

Buyers:

http://www.tf2outpost.com/trade/24054253 33 keys

http://www.tf2outpost.com/trade/24123518 37 keys

http://www.tf2outpost.com/trade/23150235 38 keys

http://www.tf2outpost.com/trade/22217321 38 keys

http://www.tf2outpost.com/trade/23138064 38 keys

http://www.tf2outpost.com/trade/24264758 38 keys

http://www.tf2outpost.com/trade/24223103 40 keys

http://www.tf2outpost.com/trade/23652342 40 keys

http://www.tf2outpost.com/trade/23830352 40 keys

http://www.tf2outpost.com/trade/23695822 41 keys

Classifieds:

http://i.imgur.com/eDuA4BT.png

Conclusion:

With most sales happening at 44 keys, I think 44 keys flat works fine. The value seems very well-established at 44 keys, with 43 keys being an appealing price to sell quickly in under a day. Sales happen at higher ranges, but not as often.