Any Harper extension would have to (a) end around his age-30 season or give him a player opt-out around that age, and (b) pay $30 million a year for his free agency years. I think that's the only way you get Boras to agree to something like that.
He's under contract next year for $5 million, so say you build in $10 and $15 million for his age-24 and age-25 seasons. That would be 4 more free agency years through his age-29 season, at $120 million? So 7/$150 million?
I think that's fair on both sides, but my guess is that Boras would ask for something like that with a player option and a bunch more years tacked on the back. If you took him through his age-35 season at $30 million per year, that 6/$180 added to 7/$150 so 13/$330 million total, with a player opt-out after the 7/$150 takes place.
And I think that's about as good as you're going to do. I would not be surprised if Boras wanted 15/$400 million.