Cost, honestly assessed
Cheaper on staples, expensive on substitutes.
Vegetarian eating is frequently described as cheaper and can be considerably more expensive, depending entirely on what it consists of. Legumes, grains and seasonal vegetables are among the cheapest food available. Manufactured substitutes, out-of-season produce, nuts and specialty ingredients are not.
The version that saves money is the one built on staples, and it requires cooking rather than assembling. That is a time cost rather than a money cost, and for households short of time rather than money the calculation runs differently.
The items worth spending on are the ones that do disproportionate work: a good olive oil for finishing, decent vinegar, and whatever savoury base ingredient a household uses most. Those improve everything and are used in small quantities.
Buying dried staples in larger quantities is the single largest saving available and requires storage and the discipline to actually use them, which is where it usually fails.
There is a cost worth naming that is not financial, which is that this cooking takes longer at the start. Vegetables require more preparation than meat, and the first months of cooking this way genuinely consume more time until the patterns are learned and the components become routine.