I have worked at an office in downtown Indianapolis for the past 5 years, and have also been on the hunt for good affordable lunch options during Indianapolis's larger conventions. There are places downtown that appear to be affordable, but end up costing you more than you expected.
An example would be the food court in the Circle Center Mall, you cannot get anything that will fill you up for under $8.50 ($2.50 drink not included), and you are going to end up waiting in a line for at least 30 minutes if you don't get there early enough (10:45am is the ideal time to break for lunch). My rule of thumb is to always opt-out of the fountain drink, they are a waste of money considering you can stock up on soft drinks from the grocery store downtown and just bring them with you where ever you go. Even abiding by my no drink policy, there are still places that I would consider pricy. Summer conventions always attract the food trucks, which are really good, but really pricy for what they offer, and almost always have a 30 to 45 minute line.
Many familiar fast food places are going to have inflated prices, not because of the convention, but because high rent and hotel surcharges are forcing these places to charge more. Any restaurant inside a hotel is going to cost more. Places that I frequent are just outside the mall. Qdoba, Chipotle, and Jimmy Jon's are $7.50 lunch and dinner options (again, don't get the soft drink with your meal). I also go to Qdoba for breakfast from time to time.
A block away from the mall past Monument Circle you can find a place called Dick's Bodacious BBQ, which features options that can get as low as $7 for lunch, but not an affordable place for dinner. Also a block away is Scotty's Brewhouse, which has a great lunch menu that averages $8, and has a bottomless soup and salad option.
I need my morning coffee and don't like the coffee in the break room or from fast food places. A grande brewed Coffee from Starbucks ranges from $2.50 to $2.90 (seems to be more expensive the closer you are to JW Marriott). Starbucks is not the only option though. The booth where Cup of Corn used to be in the mall is a new place that serves a medium (same size as Starbucks grande) coffee for $2.15 (and in my opinion tastes much better).
Hope these suggestions help anyone! If you are a local, please add to my suggestions, I may even learn a thing or two.