I'll start with what I know
rice, oatmeal, potatoes, and bananas are cheap foods. Peanut butter is cheap in terms of calories per dollar. Eating mcdoubles for the gains is a meme.
I used to own a prepaid phone to save on cell charges, but realized switching to republic wireless will save me money in the long run. look into it, but only if you have wi-fi at home. I paid $16 last month.
Instead of buying a treadmill, consider buying a jump rope. It's a lot of cardio and forces you to keep good form throughout.
Storage space is the cheapest spec of a computer and will only get cheaper. Take that into consideration when you buy your next computer.
I recently installed a programmable thermostat and will update with whether or not it's saved me any.