I've rarely really dined in Helsinki. Lack of money is one of the reasons, and that's why I prefer cooking by myself at my friends' or relatives'. That's not possible always, and if you are spending the whole day in the centrum, you can't avoid going to a restaurant.
Going to McDonald's, Carrols or Hesburger fast food chains is an option, but I try to avoid them. If I'm looking for a light lunch, Picnic Cafe is one of my favourites. It's a lunch cafe chain, and there are several of them in Helsinki. Their sandwiches and paninis are excellent, especially the one with chicken, and the food is rather cheap. Another rather popular cheapish restaurant chain is Rosso with basic restaurant food and pizza. You can have a decent meal in 60 FIM (10$) or even less. It's not highly sophisticated, but good enough.
Wow, I haven't realized until now how rarely I have actually eaten in restaurants in Helsinki... Well, I guess that's all I have to say about eating.