You have a lot of options for that, here's a few:
1. Sandwiches: You can put two pieces of bread together and add whatever you like between them, for example: a slice of cheese, hummus, turkey, chicken. You can always add veggies to this to suit your budget.
2. Salads: a salad is a very healthy lunch. A typical salad exists of: Lettuce and a mix of other, often crispier, vegetables, a source of carbohydrates like pasta or rice or potatoes, and a source of protein like chicken strips or an egg. It's easy to replace the ingredients with whatever you like.
3. (Homemade) soup. Soup is a good way to clean up leftovers in the fridge, allowing for a budget friendly meal with endless variety.

You can get very creative with all the options available, but I think these are more or less the baseline to start from and will help you eat healthier on a budget. 
Bon appetit!
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