I got down to 1 cent clicks fairly consistently about 3 years ago on Adwords. It was for a keyword with no competition, ~%25+ CTR, and a high quality landing page, advertising on the SERP only. I did something similar recently, but could not get below 3 cents, usually was around 6 cents. I think their algorithms have changed enough in the past few years to get rid of corner cases like that. (i.e. getting 1 cent clicks.)