It looks like he is leaving a bunch of money on the table by going to SJ, but I have no idea what those income taxes and expenses look like for athletes. The real world difference between what he is making for the Sharks this year and what he would have made for the Wings probably isn't as simple as the black and white dollar amounts on capfriendly. I would bet his agent does have those numbers, though, and the difference clearly wasn't enough to make Zadina stay.
We also don't know what numbers he was looking at short term and being confined to GR versus going to SJ and trying to prove himself in a bigger opportunity. The possible financial gains likely dwarf what he was guaranteed, especially if he figured to be buried in GR which would crater his next contract anyway.
In the end, he made the decision that he felt was best for him and that he was happiest with. Viewing it through any other lens probably says more of what we feel is important than what he does.