My friends went to a few SuSHi places in town and both recommended "Sushi 88" on Somerset, it's located at Somerset and Bronson intersection, right besides Gas station.
I went there once, and eatinside, with tax and 15% tip, found it still a bit pricy. If you just order take out, no tipping, that should be fine.
On Rideau Street , there is a korean garden, they serve sushi lunch as well, at good price, less than $10 I remember.
Bon Apetite ^_*