This is also mainly because the demand for salad isn't as high as the demand for burgers so it will cost more to make something that doesn't generate as much revenue. It's really messed up because if people would make eating vegetables and food without GMOs a priority to the big name brand food companies salad would eventually be
You realize that's not how it works? We have limited land and water to grow food. Lettuce happens to be a extremely difficult and expensive crop to grow. The risk in growing it and most other vegetables is incredibly high. Therefore those costs have to be passed on. We are almost at our limit of food production for the land available and less and less water is available each year.
Whereas its cheaper to make the burger because it's made of cheap processed stuff.