Looking at the price of current year refurbs, I'd say your math leaves much to be desired. Also I've stated this before, but imac used value hinges on the condition of the display. If it looks old and ugly, that kills the resale value.
For the OP, it's time to update when the computer ceases to do what you need in a timely manner. 2013 should be the next really major upgrade from intel. I can't personally wait that long, but my requirements jumped significantly in the past year (or I probably would). We're really past the days where you need to update frequently for most things.