I changed my 2011 RX350 AWD cabin filter in about the same time as you, and I also do not see why it would cost $140 to exchange one at the dealership. I purchased an aftermarket carbon filter for about $9 on Amazon.com, so the filters are cheap even carbon embedded ones. Lexus labor cost is insane.
Note: The 2011 RX350 cabin filter is the easiest to change in any car I have owned. Just open glove box, remove a panel, open the filter door, swap filters with arrow pointing up, close door and replace panel. Too easy.