Well really depends on when you want to go as the priced do vary see example below:
Norfolk Line:
Dep; 17/07/09 at 08.00 fare: £49.00
Ret:20/07/09 at 12.00 fare: £35.00
Total: £94.00
P&O:
Dep: 17/07/09 at 08.30 fare: £55.00
ret:19/07/09 at 11.45 fare: £40.00
total: £95.00
Sea France:
Dep: 17/07/09 at 08.10 fare: £57.00
ret: 20/07/09 at 11.15 fare: £34.00
total: £91.00
The fares do vary depending on time of year go next week and cost arounf £40/50 quid. Ferry takes 1.30 going over and half an hour coming back
Book well in advance and potentially get cheaper fares, or so the site says, can't find anything saying about group discounts on their sites, but have found an agent that you can do group booking through for P&O ferries
....No point in contacting them until we have dates and numbers.