The thing I like about the "idea" of running one of those things is more than just the cost factor. Shoot, setting up a van with only a $5000 investment (rounded up) sounds a lot better to me than a $20,000 investment any day, if the cleaning is close to the same. But, being totally electric (not having a heater might be a real bitch) would allow me to carry my lap-top and not have it shut down due to heat, have much more room in the van for other tools that I just can't carry now, and the equipment maintenance would have to be less than a gas engine is ... I would think, anyway.
However, the first time I ran into a carpet that needed 240 degree heat and 500 psi to clean, and required a 250 ft. hose run, I would feel like an idiot for having that EWIT.