This lunch isn't an investor meeting. It's a chance to have some food and chat about start-ups in general or general advice on your own. I'm fairly certain there is no implied chance of funding. After all, this is for charity.

That's not to say if if your idea is freaking amazing or you two click, he isn't ABLE to fund you. The chance is slim, but maybe just slightly more then meeting him on the street since he'll listen to what you have to say since your money is going to a good cause.

Either way, bid. The chance to meet him alone is worth $500 in my book.