I stayed in the Blue Horizon hotel this summer. Can't remember how much I paid but I remember it was good value comparitively. It's right in the centre of town, close to shops and Stanley Park, which is a must-see. Get a room close to the top for the views - it's a high-rise.
Not sure when would be cheapest, though.
bear in mind if you are thinking about next year, the winter olympics start in Vancouver in Feb 2010, so I would imagine, rates will be high in the run up and during the games
Went to Canada for 3 weeks in early May 2000 and travelled from Toronto to Calgary to Vancover, found this a great time to visit and very reasonable as it is deemed shoulder season between the ski-ing and sun holidaymakers. Can't remember where we stayed in Vancover except it was near Stanley Park which as Pavlov says is a must see.