Digging into it a bit here's a couple of guys who seem interesting to me.
Josh Johnson
2- 8 with a 6.20 ERA. Crap right? Well maybe not. 9.18 K/9 is pretty good. If he had enough innings to qualify(133+ IP he has 81) he'd be 12th behind Felix Hernandez. His 3.32 BB/9 is concerning. But, it's the same pace Shark is on this year and Johnson's K/9 is higher. His BABIP is really really high. He currently sits at .356. The highest pitcher who has qualified innings is at .337. So, there's reason to believe he's been at least some what unlucky. His HR/FB is also high at 18.5% which probably plays into his high BABIP. It's around 10% higher than his career average. His K/9 is around his career best. His BB/9 is a full walk ahead of his best years and obvioulsy he's giving up way too many HRs. That said, he's a very interesting buy low candidate for me. If you can get him in the $13-15 mil/year range I think he would be very interesting.
Ben Zobrist
I think Zobrist has a team option so he might be retained. However, if he isn't there is a clear hole at 2B in terms of offense. There's no clear cut option at 3B in FA that makes sense. So, unless they plan to move Castro to 2B or play Baez there its the one clear position in the infield that can use an upgrade and clearly Zobrist with a .354 OBP would be a good fit. Lake would be another possibility but he seems to be suited for the OF at the moment. Zobrist also has positional flexibility which is always a plus. He could in theory also see time in the OF.
Shin-Soo Choo
I've been on Choo for awhile because of his high OBP. But that's not the only reason. He has flexibility to man any of the 3 positions in the OF albeit his range is limited in center. The other reason is if you're looking to increase the team OBP there's not a ton of guys out there this coming off season. Granderson and Elsbury are good but you'd expect to see them back with their current teams or break the bank as the top two guys. Cruz and Byrd are interesting but big questions. Beltran may re-sign with the Cards and there's age concerns. Baker Dejesus and Crisp would all be cheaper options
They could possibly get these guys for maybe $35-40 mil. Last year they had $40 mil to spend. Given the savings from Soriano, Dejesus, and Marmol, it wouldn't be a stretch to believe $50 mil is where they would be this coming year. That'd leave them around $10-15 mil to fill out the rest of the roster. Figure $5 mil for a 5th starter in the Maholm/Feldman mold. Say $2 mil for a 4th OF in the Schierholtz mold. You'd probably need 1-2 more bench bats and 2-3 bull pen guys for the remaining $3-8 mil.