I am not an expert on this subject, but I believe there has been some dispute as to the exact economic benefit a state can derive from offering tax incentives for production. I too am skeptical of them, to be honest. I think what could be offered, in lieu of subsidies, is a friction-free environment in terms of approval and police details.
In the case of this project, there's no need for tax incentives. The producers would be missing an opportunity if they didn't choose Bangor as a filming site. If they honestly need a subsidy -- and I am being serious about this -- why not charge people VIP-like tickets to watch the filming? Why not crowdsource some of the budget? There are many ways to offset the costs of a film without resorting to tax monies.
Again, though, I am no expert. I would also point out that King could have taken care of this with the stroke of a pen -- I assume it was his call to sell the rights on this one (i.e., unlike past film projects, he owns the rights to It), so he simply could have said you need to film this in Maine.
He should try that sometime. He should write a treatment for a series called Bangor, could even be a non-supernatural crime drama for the HBO/AMC market, license it out, and then demand it be filmed in...Bangor. Just come up with a concept, compose the treatment, and then that's that. That would be cool, actually.