Sold for 500 keys.

https://backpack.tf/item/12383953068

https://backpack.tf/profiles/76561198049100395#!/compare/1758499200/1758672000 (Seller)

https://backpack.tf/profiles/76561198315558870#!/compare/1758585600/1758672000 (Buyer)

Sell listing was 500 keys, snapshot here:

https://next.backpack.tf/classifieds/snapshots/Burning%20Flames%20Burning%20Question/68d45912de1ae2d248040a40

Further elaboration on other known cases:

As of now, no trades with this Unusual have been recorded in over 1 year.

One has surfaced for circulation in the SCM, listed at $1,085.61 USD.

https://steamcommunity.com/market/listings/440/Unusual%20Burning%20Question?filter=Burning+Flames

This might have been in direct response to the previous trade for keys, but it can serve as additional reference.