I was just talking to a photo buddy who had purchased four of these Ultralast UL-BP511 Canon BP-511 Equivalent Digital Camera Batteries for only $2.49 each. He said the performance was excellent, and that opinion seems to be backed-up by 54 customer reviews on Amazon.
The battery looks just like the Canon brand (that sells for about $50 US), and has good specs. Could be a great deal. The only catch seems to be that it's only available in the US.