This sells often enough that the past month alone should be fine.

Past 30 days

3 @ 2 keys 50 ref

1 @ 3 keys

https://gladiator.tf/sales?item=Non-Craftable%20Unusual%20Taunt:%20Burstchester%20Unusualifier&at=2022-02-07T14:56:15.132Z

https://marketplace.tf/items/tf2/9258;5;uncraftable;td-30621

$5.49 / $1.83 = 3.00 keys (x2)

$5.49 / $1.83 = 3.00 keys

$5.49 / $1.83 = 3.00 keys

$5.49 / $1.83 = 3.00 keys

$5.49 / $1.82 = 3.02 keys

$5.49 / $1.81 = 3.03 keys

(Sellers at $5.49)

https://mannco.store/item/440-uncraftable-unusual-taunt-burstchester-unusualifier

$4.49 / $1.60 = 2.81 keys

$2.17 / $1.61 = 1.35 keys

$4.22 / $1.64 = 2.57 keys

https://steamcommunity.com/market/listings/440/Unusual%20Taunt%3A%20Burstchester%20Unusualifier

This sells a lot, so taking high low (excluding outliers, not countered by buyers):

Low: Jan 13, $5.22 / $2.24 = 2.33 keys

High: Feb 1, $6.09 / $2.15 = 2.83 keys

Classies: https://gyazo.com/732f6bf93938b7b894d34b33af22b83b

~2.35 key buyers, ~2.95-3 key sellers.

2.35-3 keys.