Poor student in Kansas City wants decent food!
If you like Vietnamese I would suggest Hien Vuong at the river market. Don't remember the exact price but you can get a giant bowl of Pho for somewhere between five and seven bucks. I've seen some people just come in and eat spring rolls too, they are cheap and filling.
There are a number of Vietnamese joints around the city, I'm sure most of them offer about the same prices.
Town Topic has good old school burgers pretty cheap.
Another burger/gyro place with good portions and good prices is Max's Burger and Gyros: http://maxsburgersandgyros.com/
Hope you enjoy KC.
Most of the bargains I know are in Johnson County.
Thai Treasure Restaurant in Overland Park has 3 pages of low cost lunch specials. Many entrees come in a combination meal for $7.25 including rice, soup or salad, and appetizers of the day.
Chapala is a great Mexican restaurant in Olathe. Many meals are available for under $5.
Tea Garden in Olathe has a small high quality lunch buffet in a nice setting $7 including beverage and tax.
On Wednesday and Friday, Gringo's in Olathe has a Mexican buffet for only $6.49. They also offer many complete lunches for under $5.
Bangkok Pavilion Thai Restaurant in Overland Park has a large buffet for $8 at lunch.
Ping's Chinese Restaurant in Olathe has many lunch specials at $4.95-5.50 including entree,, choice of soup, choice or rice, crab Rangoon and egg roll. Monday - Thursday they have many carry out complete dinners for $5.99.