Anything pre packaged is going to be pretty expensive. But only really when it is specifically gluten free.

We have been gluten free for DD (2.5) for about 6 months now. I have found that when you get more confident reading labels it doesn't have to be so restrictive. We still have dolmio spaghetti sauce for example, as certain flavours are gluten free. It doesn't say so on the jar but the ingredients are safe for our coeliac

Also. Some home brands are actually better than the expensive ones, less ingredients means less chances there is a nasty in there. Woolworths frozen chips are gluten free! Also Coles and woolworths have pretty good labeling for the meat products an most are gluten free. We still have sausages, meatballs, hamburgers and they are just the basic ones...

I like the macro brand (woolworths) they have nice biscuits and cake mixes for times when you need them.

It is a huge learning curve. DD also can't tolerate oats (even the certified gluten free ones). It gets easier and reading labels really saves you! I think the coeliac society also does supermarket tours to show you the items that are safe as well?