The dhal is a fantastic cheap recipe, but you don't have to soak red split lentils at all, just rinse them off first. At least, we've never soaked them and never had any problem with it.
I have a good cheap dish - salmon and rice gratin.
cheap tin of salmon (Sainsbury do a MSC certified tinned salmon for about 90p which I use and will go round four)
enough rice to go round
onion (use at least half, whole if you're feeding 4 or more)
bay leaf
a bit of flour and butter, about 1 tablespoon each
a cup of milk
a bit of grated hard cheese
Fry the onion, add the rice and the bay leaf, add water and cook till it's almost done, then drain. Meanwhile, cook the flour and butter and add the milk to make a white sauce, then add the tinned salmon to this. Pour the rice in and stir it all about, then put in a baking dish and bake in the oven with the grated cheese on top for about half an hour. You can double this recipe up and put it in the freezer before baking it, then bake it from frozen later on. Eat with green vegetables.
You can also do a vegetarian version of this using roasted vegetables. Choose whatever vegetables are cheap and good roasted (carrots, squash, parsnip, courgette, onion, celeriac, jerusalem artichoke...), roast them and stir them into the white sauce mixture instead of the salmon. The cost is about the same either way.