50p in the pound for turnover is generally the rule of thumb,however in shauns case of course he would be paying for fixtures and fittings which would be extra by negotiation, plus I assume taking over the lease which has to be factored in.
Goodwill is a bit of a grey area as it's a non tangable asset, lots of us would value the goodwill way above what others would be prepared to pay and some would simply not pay any good will at all.
If you have a business for sale and want out price it sensibly with a percentage for negotiation and move on but don't forget, Mr Gordon Brown will come into play aswell when your accountant does your final return on the business.