Typically £10 at Amazon, Argos or B&Q. (Prob £159 at Apple ;-) ) Claimed RRP £5, mainly sold at £2.99, recently £1.
Unlikely you need one if your house or fuse box is less than 20 years old, as the household circuit breakers should do the job. EDIT: Though some still recommend using these as well.
But for us who dwell in ramshackle old caves, it could be a lifesaver. (I was hogging our only one the other day for hedge trimming while my other half was gagging to do the lawn. So I narrowly avoided getting killed...)
Check out your local branch, it could be national (though spotted in Nantwich, Cheshire and posted as a local deal). Most are clearing garden stuff to make way for Halloween and Xmas tat.
PS Please don't let this thread descend into an attempt to make worse electricity puns than the one in the title. I did; now my head hertz...