The OE filters are not the cheapest thing out there...but they do work better than the replacements. I have seen some of the aftermarket ones get stuck inside the fresh air mode door and cause a god awful noise.
can easily do this yourself.
I recommend getting the Grapeseed Propylene filter that the 2009 FX50s with the advanced AC system uses. This filter is roughly the same price as the carbon and is actually a carbon filter with grapeseed fibers woven into it. Doing this yourself and buying a filter at $30-40 will save you money over the long run.
Filters like this that get replaced once a year are actually not that expensive if you look at the over all cost. You can purchase generic filters but your FX deserves better.
As a side note there are some oil filters out there that do not have the internal vibration dampener that the OE filters offer. This can cause a noise that is extremely annoying and hard to pin point. A little bit of investment pays dividens over the long haul. If you are not planning on keeping your car after the factory warranty expires.
I am not trying to nag and I do not make money off selling OE parts. However I have been with Infiniti for a while and the parts that the car was designed with always work better for long term durability and overall ride quality.