New sales:

https://backpack.tf/profiles/76561198341612787#!/compare/1528675200/1529107200

https://backpack.tf/profiles/76561198312583107#!/compare/1528588800/1528675200

https://gyazo.com/cb22955401ea544f238417f04e17acdd

Sold for ~15 keys. Keys match on buyers side (1st compare link).

https://backpack.tf/profiles/76561198312583107#!/compare/1528754400/1528840800

https://backpack.tf/profiles/76561198142978803#!/compare/1528675200/1528761600

https://gyazo.com/cb22955401ea544f238417f04e17acdd

Not all keys match (2 keys on seller's side), though it's likely that this was sold for ~15 keys as well.

https://backpack.tf/profiles/76561198119825561#!/compare/1524960000/1525046400

https://backpack.tf/profiles/76561198077724924#!/compare/1524960000/1525046400

http://www.tf2outpost.com/user/76561198077724924 - no B/O

Sold for a Strange Prof KS Grenade Launcher (~8) + B.M.O.C. (~4.5) + ~2.9 keys in taunts = 15.4 keys = 15 keys.

https://backpack.tf/profiles/76561198014273034#!/compare/1529712000/1529798400

https://backpack.tf/profiles/76561198839802490#!/compare/1529712000/1529798400

Bulk.

https://backpack.tf/profiles/76561198826120020#!/compare/1529452800/1529539200

Untrace. Though most of these hats were also traded to the latest account in the history. Perhaps friends/alts.

https://backpack.tf/profiles/76561198826120020#!/compare/1529107200/1529193600

https://backpack.tf/profiles/76561198402783260#!/compare/1529193600/1529452800

Bit bulky - better used on Tower.

https://backpack.tf/profiles/76561198402783260#!/compare/1527811200/1529193600

https://backpack.tf/profiles/76561198119634686#!/compare/1529020800/1529107200

Probably not the best sale to use here either. Would be better on the Hetman.

https://backpack.tf/profiles/76561198085813142#!/compare/1528761600/1528848000

https://backpack.tf/profiles/76561198119634686#!/compare/1528675200/1528761600

Sold for 14 keys + 16 ref = 14.5 keys = 15 keys.

https://backpack.tf/profiles/76561198077724924#!/compare/1524787200/1524873600

3 TOD tickets match in sale 1 (first ticket doesn't match, was likely used ingame since you can see he gained robo parts as well). This would make the sale 19 keys.

In the second sale ~2 keys was added, not 1 key. https://backpack.tf/item/746544660 so this sale would be ~20 keys.

https://backpack.tf/profiles/76561198269544316#!/compare/1523491200/1523664000

https://backpack.tf/profiles/76561198155320889#!/compare/1523577600/1523664000

Sold for ~14 keys.

https://backpack.tf/profiles/76561198304138933#!/compare/1526162400/1526248800

https://gyazo.com/9c9388a3555857dbf277cca1a418b6ad

Bought for ~14 keys.

https://backpack.tf/profiles/76561198304138933#!/compare/1524952800/1525039200

https://gyazo.com/9c9388a3555857dbf277cca1a418b6ad

Bought for ~14 keys.

Also worth noting that your sale 4 is from March 2017, not 2018.

https://gyazo.com/e93e5552debdb9fe0c4dc8106f495f8b

Current sellers: 14 / 15 keys

Total sales: 14 / 14 / 14 / 15 / 15 / 15 / (sultan sale, see below) / ~20 keys

I'd probably go with 14-15 keys here.

Plenty of sales at e.g. 14 keys + x ref that support this range.

I'd also like to point out that the Sultan Ceremonial was updated 3 months ago by me and was based on a single marketplace sale at 18 keys. Current sellers on the Sultan at 16 keys (including my own bot) remain unsold, which show that 18 keys is probably high, which is also why I feel like that sale shouldn't be included.

Current sellers support taking 14 into the range as well.