We can play the "pick one or two comps of varying degrees of relevance and point to how they did" game all day. If you signed 27 year old Adrian Beltre to the 2006 equivalent of 12/$240M, you'd have gotten your investment back plus a ~$75M profit by the time he retired.
Betts needs to put up roughly 46 WAR over 12 years to make that contract worthwhile if we ballpark $9M per win as an appropriate weighted average (something I believe is quite low for this timeframe). Looking at the
age 27 through 38 seasons of every player since 1970 (chosen arbitrarily, feel free to pick your own) we get 36 players who hit that mark. Beltre, who we established as a very good outcome, ranks 12th on that list.
But hey, lets backtrack a bit and get a more reasonable list of comps, at least in terms of general talent. Same years, now
ages 21 to 26. There's Mookie at #9, with 23 guys who aren't Barry Bonds within 10 WAR of him. As a group, the not-Betts-but-close cohort averaged 32.9 WAR, solidly under Betts' 37.2.
Here are
those same guys from ages 27 to 38. Five of them hit 46 WAR, and Pujols has an increasingly narrow shot at being the 6th. One Guy (Sizemore) was in the negatives, three more (Nomar, Hanley, and Cesar Cedeno) put up less than 20. The mean WAR of this group is 33.2, a total that comes incredibly close to the 32.9 this cohort put up from ages 21 to 26, would definitely get Betts into the Hall of Fame, and will rise slightly as McCutchen and Longoria continue to be disappointingly adequate.
If Betts put up 37 WAR from 27 through 38, the offered contract would likely be a modest overpay, depending on how front or back loaded those wins are and what the free agent market looks like in 12 years. How much we like Betts' chances based on his skillset, body type, modern medicine, and whatever else is up for debate, but this should at least give a reasonable sense for the shape of how his futures will likely go.