You will get more sushi for your money by going to a Korean Restaurant. I don't have a specific recco, but they alway seem to be down some side ally.
For Mex, Taqueria near portobello road is the best of a so-so lot. I was at Cafe Pacifica yesterday, very blah, but it scratched the itch. 9 quid for a smallish plate, five for a bearable margerita.