Whatever you want to earn from it, that's your rate!!
Some people will suggest that you have to charge a fortune because you are protecting the reputation of the professional photographer who has a skill that shouldn't be under estimated and needs to be paid for.
Others will suggest that as you have no experience in this kind of thing then you'd be justified in doing it for free so that you can tell all your mates that you took a photo of <insert celebs name here> and it got sent out to all X,000 of her fans.
The only correct answer is whatever YOU think your time & expertise is worth. It could be anywhere from £0 to £10,000.