You're right that it's expensive to buy lunch every day, much cheaper and healthier to make your own.
Sandwiches are good, just make up your fillings for the week, they will keep in the fridge for 3-4 days. I like tuna mayo with a bit of chopped red onion and apple, humous with grated carrot, cheese and pickle etc. Make the sandwiches the night before or in the morning- any earlier or frozen and the bread will go soggy and nasty.
Pitta pockets and sandwich thins are great, you just shove in the fillings in the morning.
It's worth buying those plastic containers where you can easily carry different foods and keep them chilled when the weather is warm , especially if no fridge at work.I sometimes freeze juice packs and put them in with the food , they defrost by lunchtime.
If there is a microwave at work, you have a lot more options!