Well this is it, for improved air quality and moisture reduction it seems a small cost. Certianly its bobbins compaired to running the tumble drier to dry clothes, and not much worse than turning the bathroom extrator timer up to two hours.Just did a bit of googling ...
I think the cheap basic models consume something like 20-60 watts, so it's going to cost you the equivalent of leaving a light on all the time, which is what, a couple of quid a month?
Daniel