http://sewelldirect.com/CarElectronics
Under safety is where there are two backup cameras one with a 2.5" and one with a 3.5" lcd, they don't give the power specs for the setup but I checked a 7"tft lcd and it was <8w so I would
speculate that it would be approximately that for 1/2 the lcd and a camera added (plus wireless coms).
Adding a "mini" PC and webcams would be significantly more power but whether the extra power consumption would be enough drag on the engine to hurt mileage more then sideviews would? I have no idea. another option would be to power the camera's or a PC from something along the lines of a motorcycle battery, reasonably light and would run a pc for a couple hours or the camera's for basically ever... but then you're adding what 10lbs/battery? 20AH motorcycle battery weighs 11lbs (just a quick google search for a motorcycle battery for a large honda bike).
Anyway! just my two cents I can see the options but I have no idea what would be the most mpg efficient I don't have the eco-modder background to support that.