Why not add other cereal crops into your rotation. They're also very very cheap - like rice, pasta, corn and flour? Add in some cheap herbs and spices to keep the raw ingredients interesting. But you do need protein too. Tinned fish is a good option. As are lentils and beans and also very very cheap. If you can, get from an Asian supermarket. You can certainly gain weight from eating these foods. You can buy cheap tinned vegetables if fresh is too expensive. And try food banks.

Lentils are lit.

Seriously, I wish I ate more of them as they're such a lovely cereal crop that just gets ignored. And cheap af too.

Hey man, i literally just answered the question in the op.

They said they have a diet that they can't or won't change for whatever reason and asked how to gain weight. The only way then would be to eat more. I can't tell them which calories to eat, or which ones they will like. They have to do that for themselves.

If they are on the edge of an eating disorder, then the best advice would actually be to get the hell off the internet and seek professional help from a licensed dietician or nutritional health expert or a therapist or psychologist or even just a PCP.

But instead, they just came here and asked how to gain weight. The only real answer is "eat more calories."