As a graduate student in search of cheap food, especially fresh ethnic food, downtown Baltimore has a surprisingly decent amount of options.
1. Rosa's Grill (Lexington Market) - I know many are people are scared to go out here but suck it up and enjoy the liveliness of the area. I've only tried a couple of items here so I'll speak on those. The pupusas (only $2 per) are made to order and take roughly ten minutes. The freshness is unparalleled as they make them in front of you. I've tried a number of items on their buffet as well which were quite good and range from $3 to $4 for a good portion size. The tamales were bad, so watch out for those. Haven't tried the other made to order items though they look great.
2. Halal food cart (corner of West Baltimore and North Liberty street) - For $6 bucks, you can get a huge portion of either chicken or lamb gyro meat on rice with the famous "white" or "red" sauces (drink included). This is all I've tried here and it's similar to what you'll find in New York City carts that made this famous. Though it isn't as tasty as those, mainly because of the cumin amount in the rice, it's something I look forward to and eat about once a week. They also have gyros and sandwiches which are cheaper (around $4).
3. Mekong Delta Cafe (105 West Saratoga Street) - So this is slightly more expensive than the previous two but much cheaper than the other options downtown. The pho here, which is the standard by which to judge any Vietnamese eatery is superb and some of the best I've ever had. They keep it real by giving you the options of organ meats but have the standard beef and chicken options if you don't want to go on the wild side. This runs for roughly $9. The portion size is huge for this. I've tried their shrimp rolls ($4), tofu noodles ($9), as well as country chicken ($9) and all were excellent. All food is made fresh and it really makes a difference.
Please suggest more if you know of any.