Assuming that you're a private seller, using the auction format:
Insertion fee = £1.30
Final value fee = £10.00
Paypal = £3.60 (or £4.10 if the payment is from another European country)
Total fees = £14.90 (or £15.40 if the buyer is from another European country)
I was going to say allow around 15% all in .. but Chris has done the maths.
Dont forget if it's an auction, you may not get what you think the item is worth... and should you set a reserve to stop the item going for a lower price...then the fees go up again, depending at what price you set the reserve.