The last suggestion was based on a single sale when there were other sales.

The range for the high-end (30 keys) will come from the sale in the previous suggestion:

http://backpack.tf/suggestion/58cd77a4e338772a275827ed

Sale #1:

Sold for 24 keys around March 5th, 2017. The Billycock took 76 days to sell.

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

http://www.tf2outpost.com/trade/28811372 B/O was 24 keys

http://backpack.tf/profiles/76561198080179568#!/compare/1488585600/1488672000

This sale was likely neglected because the buyer had a private inventory:

http://backpack.tf/profiles/76561198061449512

Sale #2:

Sold for 26 keys, this took around 2 months to sell.

Using ID filtering on the seller's inventory, 26 keys were traced.

http://www.tf2outpost.com/trade/28729786 B/O was 27 keys

http://backpack.tf/profiles/76561198262799852#!/compare/1486857600/1487289600

http://backpack.tf/profiles/76561197964171516#!/compare/1487203200/1487289600

Sale #3:

Quick-sold for 22 keys (outlier)

http://www.tf2outpost.com/trade/28775800

http://backpack.tf/profiles/76561198113922174#!/compare/1486771200/1486857600

http://backpack.tf/profiles/76561197964171516#!/compare/1487203200/1487289600

Classifieds:

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

Seller at 28 keys (1 hour)

Seller at 30 keys (18 days)

Two buyers at 21 keys

Sales seem to fit, 24-30 Keys should be fine.

Additional Proof/Counter-Proof/Constructive Criticism/Feedback is always appreciated.