The best substitutes are corn and sweet potatoes, especially if you want health with middle income budget. I prefer sweet potatoes because corn sometimes give me troubles. Sweet potatoes are perfect for me. I love them boiled or mixed in food. And I particularly love them in the morning as a substitute for rice and bread. For a big-enough size you get it for only P12 or P15. Simply boil it and eat it, you get enough carbs, fibers, plus vitamins and minerals. If you get the violet, red, or orange ones, you get a bonus antioxidant.
You can eat that with a boiled egg and a glass of soya curd or taho if you need some protein. So, P15 plus P6 (egg) plus P20 (taho) and that's just P41.00 for a really healthy breakfast. I often boil the egg with the sweet potato to save on LPG and water, too.
If you don't have P41.00 (it happens with lower middle income people), then you can boil an egg and eat it with leftover rice. You don't need to substitute rice everyday. Sometimes, you have no choice but to eat rice when you're middle income. Just make sure you don't eat too much and you burn it afterward by keeping active (not too active, though). Do brisk walk breaks now and then or climb up stairs. I often do pushup or pull-up breaks, sometimes with matching squats.