I buy the blue fiberglass air conditioner vent filters. They usual come 4 in a pack for about $2.00. Instead of changing them and throwing them away every 4 months, I vacuum them, reuse them and they are as good as new. Also, I spray a little Lysol on them to keep mold and mildew to a minimum. It works great.
Do you buy those cheap cardboard furnace/ac filters and replace often throughout the year because they get dusty and dirty? Even though they are cheap, I find that I can vacuum the dust off the filters carefully and reuse them.
Hubby has shown me how to re-use air cleaner filters by washing them out in the bathtub. Just let them dry and reuse! (Especially good for older cleaners with hard to replace filters.)