You know what? If it were 1996, and the Boston Red Sox roster looked the way it does now, I'd probably make a move for Smoltz, provided I could only give up one prospect. Now, with salaries raising everyday, there is no conceivable way, I'm going to trade three prospects or even one for that matter for a 39 year old veteran. Think about it like this, you give up Lester and receive John Smoltz in return. At the end of the season, he retires. You now have to sign a free agent to replace Smoltz. Personally, I'm a fan of Barry Zito. Let's go get him and sign him to a $12 million dollar contract. You could go the cheaper route, and sign Gil Meche to a $6 million a year contract. Still, that's somewhere in between $11.5 million - $5.5 million you have to replace by not keeping Jon Lester. The Red Sox budget is basically set at where it is now. So you're robbing Peter to pay Paul. (Solidfying the starting rotation) Granted, the Red Sox would still be a solid team, but they could be a great team if they'd paid Lester only $350,000 to do what any other FA pitcher, or Smoltz would have done for 5X less. With that money saved, the Red Sox can probably sign a FA closer, and move Papelbon to the rotation. That way, you've got a rotation that could potentially look like: Schilling, Beckett, Papelbon, Lester, and Wakefield. Maybe, the Red Sox could afford to take a gamble on Eric Gagne, or trade for a Tom Gordon.
I don't know about these kinds of prospects. In the few times we've seen him, Lester looks legit.