There's almost no accommodation in the immediate vicinity of the O2 Arena. (The Millennium Dome was built on a brown field site, well away from hotel districts).
However there's plenty of cheap accommodation available in Stratford, which is just 3 stops on the Jubilee Line from North Greenwich station (which is only 200m from the O2).
Lodge 51 has 194 reviews (nearly all of which are very good) here:
http://tinyurl.com/299nuen
Here's their website:
http://www.lodge51.com/
Their rates are here (but, as you will see, they tend to get booked quite quickly):
http://www.laterooms...._lodge-51-london.aspx
Chris