Remember what happened the last time we re-contracted a previously solid player during their first elite year? They crashed hard the next year and are now burning an injury-induced hole in the salary cap, to the tune of well over $500K. And that player was near on brownlow form in a position which we severely lack and is difficult to play; not an easy position where we are stacked.
Roberton is a HBF with a history of problematic ankle injuries. If this was his second or third year of playing at this level, then it would absolutely be worth the money we can assume he is asking. But he isn't some young gun that has been showing signs of building up to this point - probability would tell you that this year is a statistical outlier; not some miraculous skyrocket in his 'expected form'. So, not only is it likely that he will not continue to play like this, but you have the old ACME anvil dangling over the list management team's head, ready to drop as soon as his ankle flares up.
If the rumours are true about the club and the Roberton camp being some ways apart, then I would suggest that the club are probably offering a fair value. Let another club overpay, take the solid trade value, and then either fill his absence with one of our plethora of HBF options, or bring in the like of Lever via trade, or Pittard via FA.