This is the cheapest place for good-quality sushi I go to for the last (almost) 4 years. My grandma…read morewas the neighbour of some of the staff there and that's how we heard about it - at first I was skeptical (good, fresh sushi in Lisbon is hard to find at cheap prices)... then I went there to eat at her birthday. WORTH IT.
At about 12€/person at lunch (around 16€ at dinner) you get the full "à la carte" menu available to you with a wide selection of sushi, tempura, miso soup, vegetables, fried stuff (meat, shrimp, etc), stir-fry noodles, grilled meats and desserts (which I think is just fruit and ice-cream usually, it's the one thing you'll be more limited). You can order as many menu items as you want, you'll pay the same price! :)
I once even took a vegetarian friend there who feel in love with the place, so if you're vegetarian, don't worry: you'll be able to eat more than just cucumber rolls. ;)
If you're a fan of japanese matcha (green tea)- based deserts, you'll LOVE the matcha ice-cream. It's my favorite in the whole capital, since it's not so sweet that you feel like you're only tasting the sugar (which often happens since most people don't like the stuff).