When we go out in town (not that often), we usually spend a bit more, but without the drinks, there are plenty of restaurants that fit the bill near the NIA, especially Brindleyplace.
In Brindleyplace I would recommend Cielo, about 50 yards from the NIA
http://www.cielobirmingham.com/Not cheap, but if you don't go for the meat you'll be in budget (just, maybe, possibly
).
Cote Brasserie (chain restaurant, but nice) in the Mailbox is nice as well. It's just a short walk down the canal path. Lots of bars too.
http://cote-restaurants.co.uk/Cote_Restaurant_Birmingham.htmlBeing a Saturday, lots will be booked up, but you may just fancy a wander about. If you like Indian/Thai, there are lots of places dotted about. If you pull up a google map and look at Brindleyplace and the Mailbox you should find one you like.