There are currently 2 sell orders for 40 keys on the backpack.tf and 3 sellers selling for $40 on the marketplace.tf

There aren't any history to see and judge market trends for this item.

As of now, 22 of these exist in the game according to backpack.tf. Fountain of delight is in my opinion one of the best, if not best effects for this taunt because it fits the character so well.

https://backpack.tf/stats/Unusual/Taunt%3A%20The%20Mannbulance!/Tradable/Craftable/3005

https://marketplace.tf/items/tf2/31203;5;u3005

Currently all the buy orders are from bots, so they are hugely underpaying for profit. This item has not been valued before.