This has been badly in need of an update for 7 months.

Sale 1: (outlier, it was duped)

A duped one sold for 4 buds (buds were 9.5 keys)

Proof that it was duped: http://backpack.tf/item/1170143873

(4*9.5) = 38 keys

http://www.tf2outpost.com/trade/25472506

Before: http://backpack.tf/profiles/76561198062465305?time=1427094000

After: http://backpack.tf/profiles/76561198062465305?time=1427180400

Sale 2:

Sold for 47 keys (I talked to the guy and he said that it didn’t involve the Practitioner)

http://www.tf2outpost.com/trade/24918206

Before: http://backpack.tf/profiles/76561198114079058?time=1423209600

After: http://backpack.tf/profiles/76561198114079058?time=1423987200

Sale 3:

Sold for 2 buds + 23 keys (buds were 12.5 keys at the time)

[(2*12.5) + 23] = 48 keys

http://www.tf2outpost.com/trade/24884910

Before: http://backpack.tf/profiles/76561198052373942?time=1423036800

After: http://backpack.tf/profiles/76561198052373942?time=1423123200

Conclusion:

Sales at 47 and 48 keys, and a duped sale at 38. Taking the final range at 47 to 48 keys.