Financial advice company Cleo found that, after three months on the diet, meat eaters who go vegan end up spending £21 less per month on eating out and groceries.
However, vegetarians who opted to go vegan ended up spending £11 more per month.
So generally vegetarianism seems to be the cheapest diet, followed by veganism followed by meat eating.
But the research I linked to indicated that vegans generally spend less.
Also this news story cites research that says the following:
So generally vegetarianism seems to be the cheapest diet, followed by veganism followed by meat eating.