I agree with the former posters. I spent a lot of money for a headstone for my OH - it is very beautiful and enhances the lovely country churchyard where he is buried. It is somewhere for our dear girls to go and remember him; and in the main we keep it tidy and clean. But I am very aware that when I go, and when the girls move on in their lives, then it is likely to become more neglected; but in a way that is fine - it is part of returning whence we came.
I do not think spending any further money would be helpful - your OH lives on in your memory, and no amount of money spent on extra external means of remembering him can change that.
Keep him in your heart - and if there is any money to be spent, find a charity that would resonate with him, as others have suggested.