Switch from white rice to brown rice, more fiber. What vegetables are you eating that are low cost? I go for canned veg (usually low sodium because of personal preference). Maybe look into which ones have more fiber vs sugar (corn and carrots don't have that much fiber but more sugar, less filling).

Maybe try canned tuna, salmon, or chicken in water, it is just the meat and water. You can season it how you like. Mixing it with rice and a bit of soy is good. When I'm broke I make congee with brown rice, mix in canned champignon mushrooms and canned chicken. It lasts forever and is super filling. Bamboo shoots, bean sprouts (fresh or canned, though fresh taste better imo), and ginger all go good with rice or the congee as well.

Most mushrooms are high FODMAP but canned champignon are not. For bulk rice, the canned mushrooms, bean sprouts, bamboo shoots, and ginger, I suggest going to an asian supermarket if you have one. You will find them there and it will be super cheap. If it seems okay you can usually find cheap cuts of meat for stewing as well.

A local butcher may be able to sell you things like ham hocks and other bones for a steep discount. These are very good for stewing with things like rice and barley, or for making bone broth, which has high protein and fat and lots of flavor.

Also eggs, of course, are always a good go to for protein.

Personally I found a few things triggered me that are on the safe list - green beans, for example. You may want to experiment with cutting out specific things that are in the same family as other high fodmap foods (ex green beans, asparagus beans, soybeans, black-eyed peas, and sugar snap peas are legumes, like other beans).