Dried soya mince is even cheaper and can be added to things like spaghetti bolognese. Quorn pieces or mince cost more, but can go a long way. A lot of people got put off fish at school - and never go back. Just try it. If the texture is the problem, try it in a fish pie or a pasta dish.
Cheaper fish includes mackerel and herring.
Local prices vary for white fish - check your fishmonger for cheap deals. Fish fingers always go down well with the kids and they're cheap, but don't have them too often. Grill them, and slip in some vegetables, and they're healthy. Good value if you make them into a pasta dish or fish cakes. Watch the salt content though.