It's rather difficult to price this item. Nevertheless I'll give it a shot.

Last sold on marketplace.tf for $130 USD with keys median price at $1.87 USD on 9th August, 2020 (130/1.87=70.2 Keys) (Could've been a quick sell)

The preceding sale was on marketplace.tf for $179 USD with keys median price at $1.97 USD on 19th July, 2020 (179/1.97=90.8 Keys)

https://marketplace.tf/items/tf2/266;5;strange

https://marketplace.tf/items/tf2/5021;6

I also made a trade for the Strange Unusual Horseless Headless Horsemann's Headtaker with Exorcism Spell for 90 keys

https://i.imgur.com/uRufUhW.jpg

Edit: It seems the site has trouble distinguishing the variants of the Unusual Horseless Headless Horsemann's Headtaker, whether it's Strange or Killstreak, the attributes gets mixed up.