Fremantle had a super hard preseason, one of the players who has played elsewhere described it to someone I know as "brutal", much tougher than playing at this other AFL club he had played at. In my opinion, Serong's big advantage in the first part of the season was he was just fitter than his competition due to this preseason. I think this is also one of the reasons for the rise of Clark this year and why Ryan is D1 (you can see him scrambling away to get cheapies at pace etc)
So what happens during the season is that this advantage dissipates quite a bit, Serong will come back to the pack as you can't keep up these fitness levels due to playing a game each week.
That's the only reservation I have around getting him at say $630k or whatever; if those extra $ are better spent elsewhere in getting a $550k mid or whatever given Serong's pre-bye scoring is at some level built on this very high fitness base he has/had.