Thanks for this thread. Just some thoughts:
We don't ever see an example of either of them giving each other a thing, other than what they share while they are together. In fact, Jack takes one of Ennis's only two shirts.
And in later years Jack is always warmly dressed in vests and thick parka, and Ennis is in a canvas jacket. Brr. Actually even up on the mountain young Jack had that warm jacket and Ennis was in canvas.
There's a thoughtlessness to Jack around money, the way he complains about what's the point of making it, and Lureen searching for extra zeros. Ennis doesn't seem envious or bitter, he just says it sounds like some high class entertainment, meaning a high quality problem.
Another financial advantage Jack has until the end of his life is living parents with a place he can come and stay anytime he wants, something Ennis lost very young.
Jack was a pragmatist, marrying Lureen, and staying with her, enduring old man Newsome, for the security. I think he was fond of Lureen in the early years, and stayed fairly respectful of her, but I think that he stayed in the marriage for the financial security and the pretty clothes and jewelry he could thus afford.
Just thought of something else I want to add - I think Ennis really is stating his credo when he tells Junior, when you don't got nothin you don't need nothin. So partly he is truly an ascetic, but partly it's just emotionally safer to not want.