Going to UNLV you can either eat on campus which is gross, or eat at chipotle or seven eleven....Get in your car and drive down the street and your options are a little more open.
Souper Salad is right on the corner of Maryland and Flamingo and it is an option for an inexpensive lunch quickly. The quality is not great and there are other options around the same area that are I think more of a value, but great for its purpose.