Hi redmel,
I am in the same position with regard to money being tight and having a family of 5 to feed (but I'm not pregnant with my fourth lol). I found it expensive at first as I was wanting to try loads of different of recipes that added a lot to my shopping bill, but I am more selective now and making choices on cost (and sometimes speed). It takes a few weeks to get a fully established store cupboard with all the basics, but after that it gets easier.
I have found that I am a bit more selective with my choice of meals now, I.e. we may have only one or two of the more expensive meals a week (by that I mean a meal that costs more than £3.50ish for all of us). I mix the expensive meals up with cheaper ones. For example, we have pasta at least three times a week (e.g. spag bol, spag carbonara (very quick and very cheap) and lasagna with SW chips) and maybe a root vegetable stew (Iceland do a particularly good casserole veg bag for £1) with one can of tinned steak to add a bit of protein (looks gross in the can, but is gorgeous when heated through (although we do play hunt the meat hehehe)).
Red days are the most expensive and green days don't suit me as I bloat up if I have too many carbs, so extra easy is the happy medium (I just limit my pasta intake to avoid bloating). It means that as a family we can have a good variety of meals that the children mostly like. My husband and older daughter (aged 17) are happy to try most things, and I am finding that my younger children are broadening their tastes (they are 4 and 5 but still consider spag bol and pizza their favourite meals lol). However, they don't turn their noses up at curries anymore Woohoo.
If we have a chicken meal, I often buy the skinless boneless thighs instead of breast meat which makes it a lot cheaper. We rarely have sausages as I'm sure you know already, sausages for slightly bigger families adds up to an expensive meal. And as I am sure many would tell you, sauces in jars not only use up pointless syns, they are more expensive than making sauces from scratch.
Also as Brightonrosie rightly said, shopping around makes a big difference and buying veg and fruit frozen makes a massive difference too (the supermarket offers are getting better for fresh fruit). I switch supermarkets a lot to pick up my favourites from each store. Although I often have to do a quick guerilla type shop on my way home from work!!!!
I don't buy mullers as I think they are too expensive, but I do treat myself to a 500g tub of fat free greek yoghurt each week (about 80p) and add splenda to taste mixed with defrosted cherries (nom nom nom) or I mix in a little lemon curd plus a little splenda to make a lemon yoghurt (orgasmic). I buy more eggs and bacon than I used to (there seems to be a constant offer on bacon in Morrisons) and I buy the odd tub of cottage cheese to make a SW quiche, but other than that I don't find any real difference to my shopping bill (about £70 to £80 including all the household stuff, but with more effort I could reduce that further).
Sorry for the long ramble (I don't usually), but I can sympathise with your situation!! The biggest benefit after me losing weight is that everyone of us is having the recommended 'five-a-day'! Which as a mother makes me feel better!!!!!!!!!!!!