I only say £250 as I would have thought there are some wealthy people on this board that would be happy to throw down a chunk of cash for charity and to play football. Also, some of those sponsored in previous years may well have racked up a couple of hundred pounds in sponsorship and may have been confident of doing the same again.
My understanding was that you bid out of your pocket to play and once you secure your place, you then raise sponsorship in the usual way.