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honestly I have no idea... I accept gem offers for my games on trading... and every single one has felt like a massive lowball(unknown if they actually were or are fair among those who truly peddle in gems).

I did pick up Deponia complete journey(value is roughly $1.60) for 1k gems some months ago, beyond that I haven't really looked at gem prices for things.

What is the best way to calculate the price for a key in gems?

The most fair, would be to calculate its value based on where it comes from and how much it cost you. For example, if it's from a bundle, you can see how much people value it at on Group Buys. A second close option is to see it's average value on grey market sites, like Kinguin, G2A, etc.

How do I know how many gems I can ask for a game?

You can't, trading is usually a terrible experience, because most of the time, one of the parties is trying to take advantage of the other. For example, I had a key for The Last of Us from Alienware Arena, it took months of grinding and was super hard to get, but I already had the game on PS3, so, I traded it, and accepted what at the time looked like a good offer... TLOU was valued at $60 USD, never discounted or bundled, while Tales of Aries, the game I traded it for, usually went on sale for as low as $11.00 USD. But, I didn't know that at the time, so, I got basically scammed.

somethings basically don't exist on the grey market selling sites because of how long its been since it was bundled.

like Yakuza 0 for example, your always going to see the lowest price from official resellers on gg.deals(grey market are selling for over retail...)

Will say rare, but still was foundable before (on 2020-2021), for example, from indiegala feudalife challenge, get Duke Nukem: Manhattan Project, removed game, and now worth 10$ on gg.deals.

I think it the same case for Magicka: Wizards of the Square Tablet (i don't have get this one), but will say his drop rate must be around 1/100 - 1/500, and since before you can easely get 2 keys each day, i think a few players have get them during this period.