Pound for pound, beans win on cost right now, especially if you're not fussed about convenience. The others nailed it on dried versus tinned - dried is where the real savings are. A bag of dried beans costs next to nothing and swells up massively when you cook them, so you're looking at feeding a family for a couple of quid total. Meat, even the budget cuts, has gotten properly expensive by comparison.
The shift people are talking about isn't just about price though. Beans keep forever in the cupboard, which means less food waste and the ability to buy when things are on offer. Once you get past the idea that every meal needs meat as the centerpiece, the meal planning becomes easier too. You can stretch a tin of beans across multiple dishes through the week, whereas a pack of mince gets used up in one or two goes. So yeah, it's genuinely cheaper, but the real win is that beans fit better into how people actually need to eat when money's tight.