Calling Mark Teixeira's 2008 season "great" is an understatement. Teixeira switched leagues mid-season and kept on clicking, finishing with a wRAA of 46.1, unbelievably not a career high. Add Teixeira's +10 defense and the realization sets in that Teixeira is a monster of a player, worth as high as 6.5 WAR.
Heading forward, Marcels says 28.2 wRAA in 600 plate appearances. That's around 4.3-4.5 WAR (depending on your replacement level adjustment being either 20 or 22.5). From there Teixeira can dip off a third of a win per season and still be worth 175 million at the end of eight years. If you see him dropping half a win per season he's still worth 147 million. If Teixeira plays to 85% of his 2008 season, a 5.5 WAR, and dips off 0.5 annually, he'd be worth 197 million.
Take 10% off of either of those for security and you could project a worth between 160-180 million. Reports suggest the Red Sox and Teixeira are talking 8/180ish, which seems reasonable given the Sox extremely high placement on both the revenue and win curve.
Of course, there's still the matter of trading Mike Lowell. Marcels says 4.8 wRAA (a .343 wOBA, he's posted .344 two of the past three seasons) and Lowell is worth about 7 runs defensively. Even if you assume Lowell can stay healthy and get 600 plate appearances, Teixeira would still be worth more than a win in comparison. In reality I don't think they'll be that close at all.