DayZ has been available for quite some time now. However, DayZ is not exactly cheap. If you want to know the current price for a DayZ Key on Steam, you should check Steamdb.info and find out about the prices in your country there.
The SteamDB.info page provides a good overview of the current sales prices in different countries.
Buy DayZ on Steam
You can quickly get rid of 50 EUR for the Livonia Edition. (Status: 06/28/23)
Game Pass Microsoft
DayZ is also included in Microsoft's GamePass. So if you only want to try DayZ for a month, you can treat yourself to the GamePass for a month.
Bistudio (Official Publisher Store)
If you are looking for a Steam key for DayZ, the best place to find it is the official publisher store, the Bistudio Game Store.Bistudio Game Store
You can also buy the game directly from Bohemia, but it costs the same there as on Steam. The price for me on 06/28/23 is 39.99 EUR.
MMOGA is a key reseller. The game manufacturers in particular have criticized the key resellers, since they often offer keys or keys that have been acquired through stolen credit card data. That's why keys can often be offered so cheaply. Most of the time, however, keys are simply bought during special discount campaigns and then resold.
You can find a good article about the key resellers, in this case G2A, here: https://www.golem.de/news/spielebranche-g2a-findet-keys-aus-dubiosen-quellen-2005-148717.html
Is MMOGA legal?
As of 06/28/2023 MMOGA seems to be legal. The keys are usually bought in large quantities or during special discount campaigns in order to then resell them cheaper. This business practice should be legal. I have to admit that I've bought a few games from MMOGA myself. So far I haven't had any problems with MMOGA, except for one incident with a windows key that didn't work. However, whether this Windows key was legal or not, I cannot and must not provide any information here. Please note that this does not constitute legal advice. In any case, Christian Solmecke once published a video on these portals: