Buster Olney of ESPN reports today that "some rival execs view the Marlins' prospect asking price for Giancarlo Stanton as shockingly high: and "somewhat out of touch with reality."
The argument from Olney's sources: given that Stanton is owed $295 million and because he has an opt-out following the 2020 season, the price they pay in return in terms of prospect should be discounted. If they're either (a) paying a ton of money for him; or (b) potentially only having him for two seasons, they want to pay less than what the Marlins are currently asking. It's also worth noting that Stanton has a full-trade clause, so even if the Marlins can make headway in trade talks, he could veto any potential deal he'd like to
Which isn't to say that a trade isn't going to happen. It makes sense for the Marlins to ask for the moon and to see which, if any, teams try to meet them part way. It's also pretty early in the offseason, so both the Marlins and potential trade partners have an incentive to posture and try to create leverage and all of that. It's gonna be a process.
To date, the Cardinals, Giants, Red Sox, and Phillies have been linked to Stanton publicly. Figure a lot more are out there waiting to see how it all plays out.