IMO she is probably correct that canning is more expensive and I would concede the point.
Continue canning, maybe in spite of her and maybe to spite her, I'm not sure!
Relax, enjoy your canning and enjoy your MIL.
I have added these two quotes because I enjoy them. I have lots of these little thoughts posted above my computer, they help me understand why I'm me!
Mere parsimony is not economy.... Expense, and great expense, may be an essential part in true economy.... Economy is a distributive virtue, and consists, not in saving, but in selection. Parsimony requires no providence, no sagacity, no powers of combination, no comparison, no judgment.
-- Edmund Burke
"If a man does not keep pace with his companions, perhaps it is because he hears a different drummer. Let him step to the music which he hears, however measured or far away."
--Henry David Thoreau