To be fair, I question why something like this is Halloween restricted, it's quite bull

Marketplace Sales:

https://marketplace.tf/items/tf2/30532;13;strange

Sale 1:

Sold on Jun 10th for $4.48, Keys were $1.81

2.5 Keys

SCM Sales:

https://steamcommunity.com/market/listings/440/Strange%20Haunted%20Bull%20Locks

Sale 2:

Sold on Aug 30th (2pm) for $5.58, Keys were $2.33

2.4 Keys

Sale 3:

Sold on Aug 18th (6pm) for $3.44, Keys were $2.36

1.5 Keys

Sale 4:

Sold on Aug 5th (8pm) for $5.91, Keys were $2.44

2.4 Keys

Sale 5:

Sold on Jul 4th (9pm) for $6.12, Keys were $2.29

2.7 Keys

Summary of Sales:

1x 2.7

1x 2.5

2x 2.4

1x 1.5

Considering 1.5 as an outlier

Going with 2.4-2.7 Keys