Sale 1 : Sold for Power Surge Connoisseur's Cap (32) + 1.8 keys sweet

http://backpack.tf/profiles/76561198063546220?time=1447056000&compare=1447228800

With overpay, that makes 30.5 keys

Sale 2 : Sold for Blizzardy Storm Noble Amassment of Hats (23.5)+ Massed Flies Ye Olde Baker Boy (16.5)

http://backpack.tf/profiles/76561198075226743?time=1445670000&compare=1446102000

With overpay, that makes 36 keys

Sale 3 : Sold for 35 keys http://backpack.tf/profiles/76561198080795599?time=1446361200&compare=1446274800

Sale 4 : Sold for Steaming Napper's Respite (15) + Stormy Storm Thirst Blood (16.5) + 3 keys

With Overpay, that makes 31.4 keys

http://backpack.tf/profiles/76561198090073804?time=1444201200&compare=1444460400

So i think 31-35 keys is a good range. But we could consider that the pure sale is better than the rest and make 35 keys as fixed value.

Your call.