What it would take to get him
The Marlins recently traded outfielders Giancarlo Stanton and Marcell Ozuna as well as infielder Dee Gordon (who the Seattle Mariners plan on putting in center field). They did not receive great returns from the receiving teams (for Stanton, it was because his mammoth contract). However, Ozuna and Gordon should have gotten more in return. But, with Realmuto, since he is not making much salary, the Marlins should be able to get more of a return on him.
However, considering that the Marlins have spent 25 years boggling people’s minds, including, at least thus far, new partial owner and CEO Derek Jeter (somebody had to say it), they could surprise the baseball world again and get little to nothing for a trade for Realmuto.
If a trade works out for market value, I would say that the Rockies have to give up two of the organizations top five prospects (probably Riley Pint or Brendan Rodgers and one more top five prospect) and one more in the top 15 or 20 as well as another lesser prospect. If the Marlins pull a Marlins-like move, they could just ask for 2-3 mid- to lower-level prospects.