Robertson is coming off of a 41-goal, 79-point campaign and, since there are not many 40-goal scorers out there, he is going to command a lot of money. The tough part for the Stars is that many of his comparables have already signed their deals - and they're almost all in excess of $8M per season on long-term deals. What's worse is that none of those players scored 40+ goals last season.
Even if the Stars can get Robertson done for, say, 3 years, $21.75M ($7.25M AAV), they still may need to move someone off of the roster, unless of course the final year of Anton Khudobin's deal plays out on LTIR. Khudobin, 36, is slated to earn $3.33M next season.
It is worth noting that Robertson is still eligible to sign an offer sheet - and there are a few teams who can afford to pay him on a long-term deal that the Stars likely won't be able to match - at least not without moving out a sizeable contract. Anaheim and Detroit are two of the teams with both the space and the picks to pull it off. Robertson, however, would also have to be willing to sign.