If you want economy, you have to pay for it. It's a seller's market, and after Clayton Kershaw's 30 plus AAV (average annual value), the Red Sox must address the "Lester Problem". Lester's career trajectory had a speed bump in 2012, but he rebounded last season, particularly with post-season excellence.
At his best, Lester is an ace, but for most of the past five years, he performs at a number 2 level, averaging 204 innings, almost 15 wins, and a strikeout to walk ratio exceeding 2.5. As periodic readers know, K/BB ratios predict ERA better than ERA itself. The ability to strikeout batters helps pitchers escape trouble and keeping runners off base is the extension of the Sox OPS offensive philosophy.
Kershaw, of course is substantially younger than Lester, although even at age 25 he has almost 1200 innings under his belt. His ERA, WHIP, and K/BB ratio all exceed Lester's, although Lester's two World Series rings outpoint Kershaw's Cy Youngs.
Therefore, the question becomes both AAV and years. Lester's previous cancer does make a difference, as lymphoma survivors have both higher rates of second malignancies and subtle immune impairments that make them more susceptible to infection. I'd strongly doubt that 100 by 5 will get it done, but I'll speculate that an early 25 by 5 will. Gambling that Henry Owens will become Kershaw is far from a sure thing.