The cheapest recipe i know how to make is this, and my kids love it:

1 tin of woolworths brand chopped tomato, onion, garlic and basil (costs like $1)
Pasta ($1-2 a bag)
tasty cheese, grated

cook the pasta, drain and stick back into the pot, add the tin of tomatoes and grated cheese (enough so its nice and cheesy) mix it up and serve.
if you want a little extra body in it you could try sticking in maybe some chicken bits or something