I've just got back today, and while the circuit is well worth a visit, it's not like a track day when it's open to the public. I only managed one lap without a yellow flag - mostly bikers and a stuffed Merc at the Karusell. When I say one lap, anyone who's done the public day will know this; you don't get a whole lap, and you don't get the straight! We queued 20 minutes to get off the track at the end of one lap - two lane queue half a mile long. I also met the legenday tourist coaches on the same lap as virtually race prepared Zakspeed Viper - not to mention the Ring-Taxis. If I were to go again, I'd try and find a track day - I think RMA run them. As for cost - I went on the tunnel for the same sort of money being quoted in this thread for P&O, stayed at the Dorint at the circuit - expensive but if you like racing stuff, worth a visit (F1 car in the bar). Add in Fuel from London, track fees, food, drink, souvenir stickers/T-shirts etc - no change out of £700. Cost in your tyres and brake pads...!
ps (edit) don't worry about the 95dBA noise limit - it only exists in writing.