Many supermarkets do big bags of white fish fillets which are quite cheap or packs of coley or pollack fillets which are also very good value. One of my favourite ways of using these is Jamie Oliver's Coley Curry with Fluffy Rice although I use creamed coconut as one bar is the equivalent of four cans of coconut milk.
Steamed Fish with Lemon, Ginger & Chilli
I also like to steam white fish fillets as its very quick and you cook rice in the bottom tier of the steamer, veg in the middle and then your fish on top. I like to steam mine with garlic, chilli and spring onion but this Steamed fish with lemon, ginger & chilli looks really good too.