I would think that a freebie at The Amex with a collection could bring in more than £500, or possibly less but would give a band great exposure be it a marching band or standard military type brand eg Christ Hospital or similar.
Are there many local marching bands that would perhaps perform free and we allow them to have a collection before and during the game (half time) to boost their funds. I was in The Martlets centre in Burgess Hill and noticed a shop advertising one such band.