For about 12 quid for food and beer Yori offers good value if you don't want to spend too much but feel like going somewheret superior to a chain place for lunch.
Seething, shimmering intensity at White Cube; quease-inducing kitsch propaganda at the BM. Both worth seeing.
Bibigo makes for a very civilised evening out that won't break the bank.
Good value Korean around the corner from the BM ... I also recommend their Sunken Cities exhibition.