One year at $4.25 million is an absolute steal for Paul Maholm, but that's not what this deal is. The option is likely what sealed things for Maholm, and is also likely what kept the Red Sox from matching. In 2012, Boston's rotation has two holes that need to be filled (with Daniel Bard likely taking one of them), but in 2013, that's not the case. John Lackey will be back, and given he just had Tommy John surgery, should resemble the pitcher they signed (or at least the average 2010 version) rather than what showed up on the mound in 2011. Maholm will likely have his option picked up in Chicago, given the low cost and their effort to reload. Boston isn't as much of a guarantee, and declining an option that wasn't signed in good faith -- especially if a guy performs as expected -- isn't a great business practice, either.
And that's before you add in that Maholm gets to stick in the the National League, and in the division he's accustomed to, without having to fear the bats of the American League East. I wouldn't be shocked to find out that Boston's offer was similar, or even for more money, but on a one-year deal. The combination of the option and sticking in the easier NL might have been too much for Boston to contend with without paying Maholm a little too much.
Other options remain: Joe Saunders is still out there for whenever he stops demanding three years and ridiculous money, and Hiroki Kuroda claims he is staying in the majors rather than going back to Japan. With Maholm gone, expect to see the chase for Kuroda speed up, assuming he can be had for money that makes sense (and not the $13-14 million that both the Yankees and Red Sox seem wary of paying).