Port would be over a barrel in a trade needing to create the cap space. That means they shouldn't get true value re: draft picks. The value to them is the cap space. They cannot say "No thanks", they must trade if they want Dixon.
However, Adelaide would be far more attractive to Hartlett as he wouldn't need to move, plus Adelaide should be fine on cap space to accomodate assuming they lose Dangerfield.
Where Melbourne would have the advantage is to offer Hartlett a better deal than Adelaide can or want to. More years, more cash.
In effect, I don't think this will come down to giving Port a great deal on draft picks, it will more be about convincing Hartlett he wants to be at Melbourne, and then the rest will happen even if it looks like Port get ripped at face value. Melboune's second round could well be enough given the scenario.
Hartlett's brother's career was destroyed by soft tissue leg injuries, and if memory serves Hamish also had some issues a few years back. There is a reason Port have decided he is the one to go as they would have multiple options to clear cap space, medical due diligence would be key.