it's £5 at Iceland
But if you've got prime and are uber lazy - get it from amazon.
I love these because they run off of 2 AA batteries and last forevers.
I don't have space in my tiny bathroom for a charger so this is perfect.
Do note though - the battery cap WILL crack over time, and it also gets harder and harder to open as water deposits build up.
Nothing too bad for a fiver and change!