Cornflakes are GF. They're fairly cheap. A family member has coeliac. They were diagnosed over 20yrs ago when there weren't a lot of GF products available (only those on prescription and they tasted awful).
IME, generally, as long as you read the ingredients, you don't really need to buy special food. A lot of normal food is GF, even if it isn't advertised as such. (Eg, Bisto Best Gravy in jars). The only food they buy specially is pasta and bread, although corn tortillas are GF.
You can eat good, homemade food along with everyone else. Meat, veg (including potatoes) and fruit are all GF; it's more the convenience and refined food (eg, pizza, biscuits, cake, bread, etc) that can't be eaten.
Most of these are no more expensive than normal branded products, although bread can be quite dear. Eg, branded flour costs between £1.50- £2.40 per Kilo and you can buy Doves Farm GF flour for £1.70. Even Aldi do some GF free products now, and they are tasty and reasonably priced compared to their own luxury/specially selected range.
For example, when they come to dinner, if I make a roast the only thing they can't have is the Yorkshire puds, as I make my own gravy (although they can have "bisto best" instant gravy in the jars). When I make a cake I substitute a combination of things such as: ground almonds, cornflour, extra eggs and even mashed potato for normal flour. They all work well, you just have to get the ratio right.
They also don't think coeliacs should get food on prescription. They manage very well without and their food bill isn't above average for the no. of ppl.