Yes, I am also on a low income and find it really difficult to budget for the right food. I find it better rather than buying gluten-free products to simply not buy anything containing gluten at all. They always put the prices up on these things.
The other option is to try to earn extra money. For instance, I've sold stuff I no longer need on online auctions like ebay. I live in Germany and had a bit of a shoe obsession, was buying footwear from Zalando online, but realised that I'd maybe gone too far, and earned money back, by re-selling those that I didn't wear online.
Have a look at this: five dollar meals for people with allergies.