If you are getting your pricing from the Gwd website, that tariff list is for the 2011 event calculated at £55 per seat (nb not per p-o-b.) and contains a number of anomalies ... such as the PA30 which seems to have stretched to 8-seats ie £440, and nothing to do with it being a twin.
I suggest a call to Goodwood Airfield Admin on 01243 755 087 to clarify the 2012 event landing fee charge for your specific aircraft based on the number of seats (again: not p-o-b!). From hearsay I believe it may be £67.50 per seat ....
