Yes filter replacement cost is an issue - hence why i steer people away from branded & expensive brands like Blueair & IQAir unless they are loaded. Xiaomi replacement filter costs around RM 60-80 so it's actually very affordable changing it every 6 or 12 months.
Venta is not without its benefits. It's quite similar to those water filter based vacuum cleaners where they use water to trap dust.
It's good for those dusts that can be seen (PM10 above), but it still can't replace an air purifier since PM2.5 is the one that is more harmful to us as it can go into our lungs.
Humidity is best kept at 40%-50% to prevent mold & bacteria. In my experience, for my bedroom at least, even with aircon it's barely reaching 50% so actually no issue with air being too dry in air conditioned room.
When air is moist enough to the extent it won't be able to pick up water anymore, it means 100% relative humidity = condensation, that's super bad news. Though i doubt it is able to drive the humidity up to that point. But my point is, it's best to keep humidity in the range of 40-50%.
Also, i am curious though, don't you need to top up the water in Venta? I remember my Sharp 2-in-1 air purifier with humidifier consumes 100% of the water in its container in 1 single night, making me lazy to use the function after a while.
Plus, i don't quite get it. Venta sucks in air with dusts into the water, and then evaporates the water full of germs, doesn't this mean that it's reintroducing germs into the air?
Based on the above premise, if you don't clean it often (like every 3 days at the minimum which nobody would bother to), those accumulated germs in the machine are going to cause all sorts of respiratory problems after having been misted into the air by the machine.
If not mistaken Venta and other water based air filtration has a replaceable Ag ion module to disinfect the water.