Hello WriterChantal,
Others already gave so much good advice, like stay in albergues (always look for the „municipal“/ „parroquial“ ones, those are cheapest), cook, if possible together with others. If there's no kitchen, you can still eat canned stuff, cheese, olives ect. No problem once in a while. Carry a spoon and a small pocket knife for those days.
Another few things that might also be helpful to think about:
1) Stay flexible, creative and tolerant. The less money you've got, the more you need to be able to adjust to the circumstances and/or take things as they are. Remember the Camino saying „the tourist demands, the pilgrim thanks“? So true.
2) Equipment: Take what you've already got or buy second hand, no need for expensive new things, but test at home what works and what doesn't. Keep in mind if you can't buy the equipment that fits best to you, you have to adjust to what you have (more rest breaks, slower walking pace, extra shirt if yours dont dry quickly enough, ect. ect...).
3) If you are going to walk in summer, you could ask in the albergues if you're allowed to sleep in the garden (if there is one) when the weather is fine; a warm sleeping bag and sleeping mat is required then, of course. That way you'd have bath, kitchen&company of other pilgrims, but still fresh air (and the night sky and stars) at least a few times, which maybe is a compromise if you generally don't sleep well in closed rooms.
4) If you can only eat small amounts of food, make sure you still get enough calories (and salt, especially when you sweat) throughout the day without having to buy in bars and cafés. Carry olives, cooked eggs, salted nuts and raisins, chocolate... also bananas and oranges! (both healthy, not expensive, and very, very tasty in Spain)
5) Find out what food is cheap locally, don't just buy what you're used to eat at home.
6) Maybe bring a tiny, light weight cooking pot. Albergues in Galicia have kitchens, but no pots (other pilgrims will be so happy if you share the pot with them in those places).
7) Make sure you have enough money for a worst case scenario, so you can get home at any time and whatever happens.
I hope your dream will come true. Anyway, in your mind and heart your journey probably already started. Good luck and buen Camino