An excellent and rather inexpensive Mexican - some say the best in Edinburgh. Luridly decorated restaurant serving imaginative veggie fare. There's a bar next door if you really can't be bothered to move anywhere else. Expect to spend over a tenner for two courses. Barcelona, Buccleuch St Cafe Bistro with an all day menu and a big glass frontage from which to watch the world go by - outdoor tables very popular in summer.
Cheapo option is a huge plate of soup.
Inexpensive set 2 or 3 course lunches with some really interesting choices tucked away on the menu. It's run by volunteers and they have student discounts. E.g., a soup with a filled roll comes at an unbeatable 1.15. Tea and sweets are also cheap. If you don't like seafood there are other options available including veggie dishes.