Redoing this with the help of K1ng.

New price: 15-20 keys

Proof:

Sale #1 (2 months)= 18

https://backpack.tf/profiles/76561198060370030#!/compare/1527379200/1527465600

Sold for 12 keys + sweets (~5 keys) = 18 keys.

Sale #2 (fresh)= 17

https://backpack.tf/profiles/76561198095236132#!/compare/1532131200/1532217600

Sold for 17 keys.

Sale #3 (fresh)= 18

https://backpack.tf/profiles/76561198036445124#!/compare/1532044800/1532131200

Sold for 18 keys.

Sale #4 (1 month)= invalid

https://backpack.tf/profiles/76561198123248183#!/compare/1529712000/1529884800

Sold for a steaming ye olde baker boy. Not gonna mini. It has an unsold for 7 months at 15 keys, making it 14 keys at best, which is below buyers.

Sale #5 (2 weeks)= 16

https://backpack.tf/profiles/76561198328419180#!/compare/1531008000/1531094400

Sold for 16 keys.

Sale #6 (1 month)= invalid

https://backpack.tf/profiles/76561198168863803#!/compare/1529625600/1529712000

Sold for a steaming brass bucket + australium frontier justice (9). Better used there, since unpriced and only 5 in existance.

Sale #7 (2 months)= 16

https://backpack.tf/profiles/76561198370713049#!/compare/1526256000/1526342400

Sold for 16 keys.

Sale #8 (3 months)= invalid

https://backpack.tf/profiles/76561198156512874#!/compare/1525737600/1525824000

Sold for a tf logo airdog. Hard to trace, but i think the buyer has bought the airdog just before trading it for the shark hat, making the buyer not appear in the airdog history. Unsure tho. Excluding.

Sale #9 (3 months)= 15

https://backpack.tf/profiles/76561198042664706#!/compare/1524614400/1524700800

Sold for 15 keys.

bot.tf sales:

https://bot.tf/stats/item?def=30800&q=5&ef=Starstorm+Slumber&craft=1&aus=0&ks=0

https://imgur.com/a/X8ahJST

https://backpack.tf/profiles/76561198344933095#!/compare/1524520800/1524607200 - Untracable = invalid

https://backpack.tf/profiles/76561198344933095#!/compare/1524866400/1524952800 - Untracable = invalid

https://backpack.tf/profiles/76561198453417865#!/compare/1526076000/1526162400 - 16

https://backpack.tf/profiles/76561198453417865#!/compare/1526162400/1526248800 - 3 keys missing. Can't trace them. = invalid

https://backpack.tf/profiles/76561198386441509#!/compare/1526428800/1526515200 - 18

https://backpack.tf/profiles/76561198382589541#!/compare/1526594400/1526680800 - 17 keys 10 ref = ~17

https://backpack.tf/profiles/76561198453417865#!/compare/1527285600/1527372000 - 19

https://backpack.tf/profiles/76561198382654640#!/compare/1530309600/1530396000 - Untracable. Looks like private inventory = invalid

7 quicksells at ~15 keys, 1 at ~16.

Marketplace.tf sales:

https://marketplace.tf/items/30800;5;u110

Sale #1 (2 months) Sold for 39.99$. Keys were 1.97$. 39.99/1.97 = ~20 keys

Sale #2 (2 months) Sold for 45.00$. Keys were 1.94$. 45/1.94 = ~23 keys

Sale #3 (2 weeks) Sold for 25.01$. Buy orders above this = invalid

_________________________________________________________

SUMMARY

Sales:

15 x8

16 x4

17 x2

18 x3

19 x1

20 x1

23 x1

Buyer at >15, but doesn't counter 8 sales.

23 is high.

Going with 15-20 keys.

Feel free to add proof to support or counter my new price.