Usually the more expensive of choices like YO! and Kulu Kulu, if you bide your time and get to the right Itsu shop then you'll get a sushi dinner for less than both of these.
The Itsu sushi restaurant in the N1 Centre takes 50% off all their sushi half an hour before they close. Most days this is 22.00 so we would suggest that you have a drink or two after work and then grab a cheap sushi dinner to take home!
Just what we need as money turns tight, you can buy some wrapping paper and Christmas baubles in Paperchase while you're at it - handy.