Yes and no. He cannot make up his own mind, because the number of years available to him to sign a high-priced long term deal probably equal the number of fingers you have on one hand (assuming you are not a bad carpenter).
By holding out his year and if there is no football next year, Mankins just cut off two fingers.
And while it is true that if he managed his money properly he doesn't need any more, that is irrelevant to whether or not he is in control of his destiny or not. All the money in the bank does is make not controlling his destiny not hurt so much. He still has only short-term control, and when all is said and done, may not in the long run make more money than he would have had he signed the Pats offer.
But as Breer keeps saying, with Mankins it isn't about the money. It's about the principle.