Calculating Back-Up Drive Capacity

Not sure if this is a valid question to ask. I am trying to determine an adequate storage capacity for a back-up portable HD. I use a Canon 400D & a Canon G11 to shoot RAW. Although the size of RAW files can vary, is there any basic rule-of-thumb regarding the average RAW file size? If so, I can get a rough estimate of the storage capacity I will need.

Thank you.