No idea what it would take to acquire him, but he'd absolutely be a great addition. He has a club option for $7.5 million next year which is absolutely dirt cheap for someone who brings the value he does. Even if his skills decline a bit, he's going to bring serious surplus value.
Of course, the Rays know this and they're notoriously difficult to trade with so I won't be getting my hopes up. But if he does come, here's a look at what he brings to the table:
He's played over 150 games for five straight years, and over 155 for three straight. He's a guy who is going to play almost everyday.
He's played five full seasons in the bigs and been a serious asset at the plate in four of them. His down year was 2010 when he was basically an average hitter (weighted runs created of 100 which is perfectly league average). The last year and a half is most relevant, and in that time span he's gone from a fringe MVP caliber player to just an above average one. His numbers last year and this are remarkably constant: 2013 he had a .756 OPS, this year he has a .753 OPS. That might not seem great, until you remember the M's regularly start at least two OF's out of a bunch with numbers like this: Ackley, .617, Chavez, .651, Jones, .670. Zobrist is clearly an upgrade to the OF both for this year and next.
What sets Zobrist apart is his defense, both his ability and his flexibility. He can play anywhere. Not in the Willie Bloomquist "can give you passable defense at a variety of positions" sort of way, but in a "this dude could really start at any number of positions" sort of way. And he does. Just in the last year and a half, Tampa has started him over 40 games at shortstop, over 170 games at second base, and 59 games in the OF corners. If we got back further we find him playing CF and 1B and even a sprinkling of games at 3rd.
Of course, we don't really care about all that, because he'd only get a handful of starts in the infield here. We'd be bringing him in to play a corner OF spot, and as a corner OF the last two years he has a UZR/150 of over 10, meaning he's been worth more than 10 runs above average per 150 games. That's not gold glove caliber, but it's well above average, and if you prefer to have larger samples for defensive numbers his career UZR/150 is 18, which is absolutely gold glove caliber. So he'd be at least Ackley's equal (Ackley's quietly turned into a very good defensive outfielder) while being a serious upgrade over Chavez (or Romero or Gillespie or whoever else).
And of course, he gives you insurance in the horrific event of an injury to one of the infielders.
Despite the decline from his lofty 2011-2012 numbers, Zobrist's total package is still going to be worth around 5 wins this year. Let's say he doesn't play any shortstop or second base in Seattle so we ding him one win for positional value, he's still a four win player, which would make him our third best position player by a country mile. More importantly, that's an upgrade of at least three wins over anybody else we have, and it comes with the confidence that he's very likely to stay healthy. Add in that he's massively underpaid this year and next and he's the perfect acquisition.
Again, I don't know what he would cost, and I'm skeptical he can be had for something reasonable. If Tampa's legitimately shopping him they're going to get some very strong offers. Talks have said to include Nick Franklin, but I'm sure a deal would either include another couple of strong prospects or a stud prospect like Walker. Not sure I'd do Walker and Franklin for him but I'd think about it long and hard. I'd definitely do Franklin, Ramirez and any of our relievers, but I'm sure that's not enough.
In conclusion, I doubt we acquire Zobrist, but if we do I'll be pretty excited, as long as we don't completely mortgage the future to do so. He'd be a major difference maker here.