The grey market is weird like that. If they have a surplus of keys and the games a couple years old, you can get like 12 for the price of 1 full priced copy. When they only have 1, you get weird prices like this. It is literally the only key I found. Usually when theres a limited amount of keys, its a dollar or two more, not a 50% mark up. For American prices, you're usually better off just buying it on Steam in this scenario, but it may still be a "deal" elsewhere. Steam has all kinds of odd regional limitations, so it probably makes sense somewhere in the world.
The fact a single key is all that exists is pretty telling of how little anyone cares about HB. I can usually find keys for all kinds of obscure and garbage games with minimal issue, not this one.
Now all of the sudden, it's $16. Almost makes me wonder if they run crawlers or something to automatically adjust prices based on shit like how often a game is searched on grey market sites.