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And if Alonso were to turn out to be a player of that level, but healthier, that would be a nice return on Lee. You'd have the 3+3 years of value that you mention.
Johnson had one of the most freakish BB rates of recent years, but OTOH his PWR never projected to much. No reason to rule out 25-30 homers for Alonso yet.
As mentioned, if you're getting 6.5, 7.0 runs per 27 out of a first baseman with 6 years of club control, that's a player to shout about. Alonso could easily deliver MOTO production very soon after making the bigs.
Looking at it from a Nick Johnson MID projection, the trade looks better to me. Especially if you could finagle hi-upside young players in the deal.
Maybe Jack could ask you to pick him out a Robles, G :- )