I used to own one of these so thought I'd update the 7-year-old price. I traded mine for 20 keys (didn't take me more than a month on bp.tf classifieds to find a buyer):

https://backpack.tf/item/1792090675

https://backpack.tf/profiles/76561198032578836#!/compare/1735084800/1735257600

Had a look at the below ID history which has been traded recently:

https://backpack.tf/item/15143600164

I think this was traded for ~21 keys in late November 2024 (the key histories match). Maybe 30 refined was involved but histories not logged?

https://backpack.tf/profiles/76561198838524661#!/compare/1732320000/1732406400

Then traded to stntrading bot same day. I can't decipher what the deal was.

https://backpack.tf/profiles/76561198065639920#!/compare/1732406400/1732492800

The bot is currently buying ~15 keys and selling for ~22 keys. Unlikely someone would trade it same day for less than what was paid?

https://stntrading.eu/item/tf2/Unusual+Kill-a-Watt+One-Man+Army

https://imgur.com/a/1Cgm7aY (buyout currently sitting ~22 keys).

Might be other recent sales but I don't have bp.tf premium to check them out.

This is just supplementary evidence of older sales to support the notion that the current 13 key suggestion is too low. If you login to MP.TF you can see no sales recorded for 2024. You can see sales between $34-45 USD in 2021-2023.

https://marketplace.tf/items/tf2/601;5;u56

I won't be doing another suggestion for this just because it is a few % lower than it could be. Anyone reading this is welcome to interpret these sales as they wish.