Eating lunch out every day can definitely get expensive, and people don't realize that. Of course, you don't have to buy your lunch in order to not eat at your desk. Assuming you have a place you can go (in this guy's example, a park), you can always bring something from home. Or, if you need something more exciting than what you can make at home, do like this guy:

http://www.slate.com/articles/life/food/2012/02/liberate_you...

...and then bring it somewhere other than your desk to eat.