I would not recommend eating only bread for two or three days and then having a pilgrim meal. I think you would be far better off (if money is so tight) avoiding restaurants and buying cheese, salami, nuts, a carrot, greens, yoghurt to go with your bread every day. This would be much more healthy and just as cheap, especially if you stock up in supermercados rather than relying on tiendas.
Another strategy would be to offer to cook for other pilgrims and ask them to contribute towards the food cost - we could make a good dinner for five for ten euros. Pasta, tomato-based sauce with onion, garlic and grated carrot, maybe mushrooms, chorizo, olives, cheese, green salad, egg, melon.
Another budget option would be to soak half a cupful of rolled oats in a pottle of yoghurt overnight (you'd need to carry a small lightweight bowl) and eat that for breakfast. More nutritious than bread alone and good slow-release energy.
With a little planning *healthy* and *cheaply* need not be mutually exclusive.