First, a quick refresher on the process for Japanese players, whose access to the major league market is controlled by their clubs - in Maeda's case, the Hiroshima Carp. We saw this last season, where the team decided not to put Maeda up for auction, instead opting to hold onto him. This was, I suspect, in part due to the change in rules concerning posting fees. Prior to then, there was no "cap" o the amount an MLB team could bid, and all that money went to the team, so it was in their interest to maximize a player's value. Now, there is a $20 million maximum, so Japanese teams will want to get as much out of their asset as possible, before cashing in.
Any time can now put forward their interest in Maeda, along with a post-dated check for $20 million. [There's little doubt you will need to pony up that in order to be in the conversation[ When the deadline passes, Maeda and his agent are then free to negotiation with all of the teams which entered the fray. Whichever team they decided to sign with, pays the $20 million to Hiroshima, as well as any costs associated with Maeda, e.g. contract, signing bonus, etc. The other teams get their checks torn up and can walk away.
The Diamondbacks have made no secret of their interest in Maeda, and with the rapidly declining pool of high-end free agents [David Price and Jordan Zimmermann has gone, Johnny Cueto has turned down a Diamondbacks offer and Zack Greinke is expected to sign with a division rival shortly], Maeda becomes significantly more important to the team. Hard to say what kind of contract he might get. I'll probably write up Maeda in more detail over the weekend, but MLB Trade Rumors projected a five-year, $60 million contract, on top of the posting fee, while Fangraphs capped things at six years and $85 million.