Marcia Vanderlip, food editor/writer for the Tribune, has a really good listing of good places to lunch downtown for around $5. The article is here.
I thought I would throw in a couple extra comments. Formosa is good; the other underrated Chinese restaurant is Jingos, next to the Regency hotel. The prices there are in the $7-$11 range, but the food is excellent. I find the food at Jingos to be a little ligher and more nuanced than Formosa but both places are excellent.
I think Cherry Street Wine Cellar and Bistro also is a place that is often forgotten for lunch. Lunch there is not an extravagant affair; again, prices are extremely reasonable, ranging for a $5 salad to a $15 entree; I personally prefer the Camembert sandwich. The menu is here. Given the quality of the food, this might be the best lunch value in Columbia.
The Indian restaurants downtown (Taj Mahal) are decent but unsatisfying to my southern Indian palate. The food is good but I find it somewhat bland; this is because Indian restaurateurs cater to the bulk of their clientele (aka non-Indians) which means toning down the spices and presenting simple, accessible food.
Other places I stop by semi-frequently: the Artisan for simple, wholesome panini sandwiches, Sub Shop, Sycamore, Teller's.
In the works: My perspective on restaurant winelists around Columbia.