Simple, cheap, efficient, no frills and delicious. No fancy pants, Western-style appeal like Banh Mi Boys. $2-4 is about the most you can expect to spend if you have a normal human stomach.
If you're not so into the the banh mi, they have various kinds of prepackaged Vietnamese spring rolls so you can grab and go. The overpackaging (styrofoam AND saran wrap) make me a little queasy but I guess it's the price you pay for convenience!
A great place to run in on the way to class, an appointment, to the subway, streetcar, etc. etc. You can be in and out in 5 minutes. Love it!