My DD is coeliac and I wouldn't have said I've noticed a massive increase in our shopping bill.
I pretty much cook everything from scratch and don't eat lots of things that would contain gluten anyway, so stuff like shepherds pies, chilli, etc. I buy GF pasta, but none of the other GF products and make cakes, bread, etc myself with GF flour.
The flour is more expensive than normal flour, but nowhere near the price of a pack of GF cakes.
We get a prescription for the bread but DD hates it so I bought a bread maker and make GF bread, pizza dough, etc for a fraction of the cost of GF bread/pizza.
We all tend to eat the same so I don't have to worry about contamination of making lots of different dinners, so I make pastry for pies with GF flour, GF suet and flour for dumplings in a stew
The flour, suet, etc, can be more expensive but nothing like the cost of "Free-From" type stuff