While you can't avoid spending money on a business lunch with a client or prospective client, you can save money on making your lunch at work every other day. Below is the strategy I use to avoid spending $20 to $50 per week on restaurant lunches.
At work in the fridge, keep a lunch box, like this one from California Innovations that I purchased at Kroger. Inside, store mayo, lunch meat, cheese, cream cheese, and anything else you need to create your meal at work. I also like mason jars filled with lettuce and toppings for a great salad. I keep cream cheese for my morning bagel too!
In your desk, keep a bag of bread and chips or crackers. I keep pretzels in a plastic storage container in my desk drawer. This is much cheaper than the $1 tiny bags of chips in the vending machine. My bread bag, as I call it, also has a bag of bagels for those hurried mornings that I miss cereal at home and can't be late to work. Knowing that I have something waiting for me is a big help.
Lastly, don't be shy to take free things left in your office kitchen after a meeting, like granola bars, cookies, or something similar. Just don't take all of them. Keep some in your desk for snacking later or to accompany your lunch.
This week, I spent $16 for all the items I used for lunch this week. I assemble my lunch in the kitchen or in a private conference room or at my desk. Your work kitchen likely has plastic utensils and paper plates, like mine, so help yourself.
If you make your lunches at home or work, you will have plenty of money left over to treat yourself to lunch with a friend once a week. Let me know what you do to save money on lunch.