Your main objective
1. cheap food that
2. maintain a healthy calorie intake

So I will suggest base on these 2 points and disregard the rest.

What to eat
1. Chicken breast (cheapest meat with condense protein)
2. Rice (cheapest carb compared to bread, noodle)
3. Cheapest vege you can find in grocery (vege price fluctuates, just pick the cheapest per kg one)
4. Drink only plain water.
5. Drink only beverage found in office pantry.

Cook your own
1. Gather a few aunty, maybe 10, buy meat in big bulk and bargain for discounted price.
2. Buy the price cut vege / meat from Tesco, check if they are still in good condition.
3. Store your meat in office fridge (no electricity cost involved)
4. Buy a rice cooker, bring it too office and use office electricity to cook your food (no electricity cost involved)
5. To further lower the cost to maximum, add no seasoning, just boil it. Steal Mc Donald's salt / pepper / chilli sauce / tomoto sauce and ciao if you want/dare.
6. Use office dishwasher

Eating out
1. Only buy meat from zap fan stall, take the sauce as it wont be counted to the price
2. Cook your own rice in office
3. Use office dishwasher

Lastly
https://goodyfeed.com/7-ways-get-free-food-...apore-malaysia/