Just go with cheap ones and replace every three or four months.
The primary purpose of the filter is to protect the fan and heat exchanger from dirt and dust. It's not to help "clean" your house's air. Your furnace fan does not produce very much static pressure to pull air through a filter; the thicker the filter, the more it's going to reduce your furnace's air flow, which will become a problem eventually, potentially freezing your A/C coil or cracking your heat exchanger.
MERV 6 is plenty.
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