That is all true, but even though there's better options, it's hard to have all the best mids.
I think that i would be happy with him as an M8 at the end of the year.
Sorry that's terrible logic. You don't want a 100ppg player as M8 by the end, even if slight unders. You want the best possible. There are always players who are improving and you need to jump on their wagon and get them right.
To look at your scenario there, Mids who started the season under $500k but did better than 105ppg include:
Dayne Beams $497,200 115.5ppg
Robbie Gray $442,600 111.1ppg
Luke Parker $468,800 108.5ppg
Nathan Jones $485,700 108.1ppg
Adam Treloar $469,500 107.5ppg
Dyson Heppell $491,400 106.4ppg
Marc Murphy $498,300 106.1ppg
Those are the sorts of players you need to look for next season at the price you suggested, not Sam Mitchell.