I'll retract that he's over paid because 1 I don't know what he's getting paid and 2 I don't know what others of his output are getting paid. But my original point still stands that experience and talent shouldn't come into contracts it should be base on output. Of which I think Mackay hasn't lived up to his ability.
In terms of output, Mackay deserves about the same contract as Brodie Martin. Potentially his ceiling is higher, however he's proven that he's not capable of playing near it for extended periods of time. If someone said that one of Martin or Mackay is going to another club, I doubt many would care which one it ended up being.