A heating and air expert told my friend that the expensive pleated air filters actually cause your HVAC unit to work harder , thus using more energy and causing it to have to be repaired more often. He suggested she use the "el-cheapos" and replace them more frequently. I was a little skeptical until my mother-in-law told me her HVAC repair man told her the same thing. I emailed our HVAC expert and he said we should definitely not be using the pleated filters but instead he uses poly filters, which can be found at HVAC supply stores. He said to certainly never leave a pleated filter in for more than 30 days.
Wish I'd known this 4 years ago when I started buying those $12 air filters that filter out all the gook nobody wants in their house! Two days ago I put in the inexpensive filters (used to be blue but are now green and made with recycled materials). You can get a 4-pack for less than $3.
But I do plan to look into getting the poly filters down the road. Just need to make a trip to our local HVAC supply store!