The key to a good Raw diet is variety . . . . .. Tripe and chicken carcasses are the cheap way of doing it, however i feel you need to add extra meats to give a balanced diet.
Ask your mum to get the butchers to keep the left over trim, some fat, beef bones, etc. I would buy lamb breast a couple of times a week, a whole breast should do 2 meals.
That coupled with some reduced items from the supermarket, mince, liver, kidneys, fish, chops, steak, go on a sunday and you'll be able to get loads.
In terms of cost you need to look at the quality of the food your giving, yes if you add