Well, it's dependant on what you want it for. People have reported that the Pronto is frugal with power, so you're not replacing batteries all too often.

However, the dock does make a nice place to keep your unit when not in use. Plus it's comforting to know you've always got a full charge and you'll never have to worry about replacing four AA's.

But if you're thinking of using it to save money, I don't think it'll pay for itself over any reasonable timespan. It's more of a convenience/decorative item.