Don't know about the club class cabin and time of year but we usually pay around £300 including the 7 nights camping cheques and a 4 berth outside cabin (only two of us though) going to Rotterdam for Dusseldorf we payed £370, Possibly the prices are rising.
It's a good price for us. Cost us £700 in September without the camping cheques. Just checked the price on p&o site and it was over £600 for the trip we have booked with no cheques. We are 8.7 metre van though.