After 3 years from being drafted, it's no longer based on the rookie contract. They'd have to negotiate a 3 year deal with him, but it's really just Utah and Barcelona competing for his services.
The buyout should be reasonable (since it's an option year), but the Jazz can only contribute a portion to that. Anything else, and the contract has to be large enough to cover the buyout and pay him at market value. He's been making about $3.5 million per year (or so), so the full MLE of $5 million per year (which is what they pay Booker) should be enough of an upgrade to make him consider it. If they have cap space, they could go above that number - if that's what it takes to get him to come over. The main reason that he'd consider coming over is his earning potential. He'll need a real commitment from the Jazz to get him to come over, but the value of his 2nd contract (if he continues to play well) dwarfs his initial Jazz contract, which should be about twice what he's making right now. NBA teams simply pay better for proven talent.
For refrence, Nicola Mirotic just paid $3 million for his buyout, but then got a 3 year $17 million contract from the Bulls. Mirotic is essentially in the same boat that Tomic was, except the Jazz weren't ready to bring him over when they had a chance and he signed a 3 year (plus 1 year team option) contract with Barcelona when he was a little older than Mirotic is now. At 28, this might be his last shot at NBA money if he doesn't come over this year.