David Bernstein writes that the best deal for vegetarian fast food is the Taco Bell bean burrito at 79 cents.
I haven’t set foot in a Taco Bell in several years, myself. But if you are fortunate enough to live in a city with a Back Yard Burgers franchise, I highly recommend their Gardenburger. BYB is a little more expensive than the usual fast food dreck, but this is definitely a case of getting what you pay for.