Gladiator.tf stats: https://gladiator.tf/sales?item=Non-Craftable%20Unlocked%20Cosmetic%20Crate%20Engineer&at=2021-11-14T08:57:29.436Z

Prices aren't fresh for suggestion, since nobody buys or sells for those prices.

Marketplace.tf: https://marketplace.tf/items/tf2/5858;6;uncraftable

3 sales between 4 November - 9 November for 2.99$

November 4 sales: 2.99$ / 1.87$ (November 4 key price at marketplace.tf) = ~1.6 keys

November 9 sale: 2.99$ / 1.88$ = ~1.6 keys

Mannco.store: outdated sales

Steam Community Market: https://steamcommunity.com/market/listings/440/Unlocked%20Cosmetic%20Crate%20Engineer

First sale (October 15): 2.78$

Key price for First sale (October 15): 2.17

2.78 / 2.17 = ~1.3 keys

Second 5 sales (October 17): 2.78$

Key price for those sales: 2.22$

2.78 / 2.22 = ~1.25 keys

Third sale (October 18): 2.78$

Key price for this sale: 2.23$

2.78 / 2.23 = ~1.25 keys

Fourth sale (October 20): 2.81$

Key price for this sale: 2.25$

2.81 / 2.25 = ~1.25 keys

Fifth sale (October 22): 2.81$

Key price for this sale: 2.19$

2.81 / 2.19 = ~1.3 keys

Sixth sale (October 24): 2.81$

Key price for this sale: 2.24$

2.81 / 2.24 = ~1.25 keys

Seventh sale (October 26): 2.82$

Key price for this sale: 2.24$

2.82 / 2.24 = ~1.25 keys

Eighth sale (October 29): 2.81$

Key price for this sale: 2.20$

2.81 / 2.20 = ~1.25 keys

Ninth sale (November 6 (I didn't count quicksell as a sale)): 2.76$

Key price for this sale: 2.23

2.76 / 2.23 = ~1.25 keys

Tenth sale (November 8): 2.79$

Key price for this sale: 2.19$

2.79 / 2.19 = ~1.25 keys

Recent sale: 2.76$

Key price for this sale: 2.12$

2.76 / 2.19 = ~1.3 keys

Listinings: 2.78$

Key price: 2.14$

2.78 / 2.14 = ~1.3 keys

I guess low value will be ~1.3 keys, because there is some sales for this price. I know there is buy listinings for ~1.2 keys on backpack.tf. But! no verified sales.

(Bonus from me: This was really painful. I lost 1 hour on this suggestion, because my currency is RUB. It was my the longest ever suggestion. I hope it won't get cancelled)