I *think* it's 20 Euro per bill, I looked it up on their web site about a week ago, so it's up there - can't quite remember it off hand. Not sure about installation costs - I remember thinking it didn't apply to me, so they had something about it up on their site.
Whether it's worth it - you need to work out how much energy you use and how much you could transfer over to the night band.
Don't forget water - can have it hot for when you get up in the morning.
It's notably cheaper - however, this is dependent on your bills. Have a look at 1. your bills, and 2. the appliance calculator on their web page which will give you an approximation of the amount of energy standard appliances use. Work out what you'd be able to change over to the night rate.
- Worked out, not worth it for me - but I'm generally a low energy user. Putting on a wash in at night isn't suitable. In fact at night, everything is off, so it's only hot water I'd get from it.