There are several NFL franchises whose revenue generation and Value are lagging significantly behind others' (among them the Chargers and 49ers). For teams like this, it would make economic sense to pay Belichick a ridiculous amount of money (north of $10 per year) if they were convinced that Revenues would grow and their own value would increase as a result.
The Patriots have a very high Value and are generating substantial revenues, but their Operating Margin lags because of the debt incurred for the purchase and/or improvement of Real Estate, most notably, but not solely, the Blade. So, there might be some internal limit that the Krafts have put on what they might pay BB (same goes for Pioli).
So, if it's a purely economic decision, it's not 100% certain that the Patriots keep Coach.
However, I'm one of those who think that money isn't BB's only motivator, that he sees a chance to be considered among the eternal coaching elite (Brown, Halas, Lombardi) by sticking with the Patriots and that the Krafts just have to continue to be reasonable and give him the tools to win in order to keep him in New England for the rest of his career.