The most famous dish in Sri Lanka is the Sri Lankan rice and curry, made with rice, meat and various vegetables cooked with spices. Kottu is also very famous (served only for dinner) and consist of chopped roti cooked on a hot plate with spices, vegetables and meat (or eggs).
The food in Sri Lanka is spicy and even the locals are known to find it so.
If you don’t want spicy food, just ask for “non-spicy” when you order. You can also try Lamprais: a delicious (mildly spicy) Dutch influenced mixed meat curry, with meatballs and caramelized onions, wrapped in a banana leaf.
It is hard to find either pork or beef in traditional restaurants due to the religious reasons. However, you can find a lot of vegetarian restaurants and chicken and seafood is freely available.
There is also a tradition of eating short-eats, which are delicious snacks like stuffed rolls or little chicken pies.
A traditional Sri Lankan meal in a small local restaurant will cost between 200 and 300 Rupees.