What’s your typical monthly maintenance cost?

[MatthewSmedo]

Currently I'm in the research stage and trying not to jump into a decision. There seem to be so many brands available that it's easy to feel confused. If anyone here has previously been through all of this, what factors ended up your most important when making your final decision?
 
https://southwestspro.livejournal.com/1994.html
 
How frequently do you replace filters? 16344bd