• One important metric: m³ / (hour*$), so how much it can filter for how much money
• Also the volume on the setting that gives the filtration power needed. Often it is best to get a bigger one, run on middle or low level, especially for office or bedroom.
• How much m³ / hour overall? When it is against dust, allergens, pm2.5 etc., filtering your room volume once per hour is decent. To protect from viral infections, e. g. at a dentist or doctor, 6x the volume of the room is ideal. For private use, it’s nice when it can do once per hour on a lower setting, and for occasional parties 6x of that on a high, loud setting to avoid spread of viruses.

Pretty good for your money is the Corsi-Rosenthal Box mentioned already. As for things that don’t require assembly, Trotec beats all prices in Germany, e. g. the 250E or 350E: https://de.trotec.com/shop/design-luftreiniger-airgoclean-250-e.html That would provide more than you need for typical home use already. For a single room such as your bedroom, you can get something really decent for less than $250.

The ones that are below HEPA standard are not as bad as somebody mentioned, imo. Against many things, such as dust or allergens, they should be fine. I’m buying only HEPA filters myself, though; doesn’t really save much otherwise.