[He doesn't know the significance. His memory has yet to be jogged to be reminded of all those t-shirts Will bought, and how much they cost, despite knowing that that occurred.]

Simply because it's a lot to spend. She's very careful with her own money, although I'm betting the only reason she's here is because one, she was asked to be, and two, because the money would be for charity. I'm just giving you the forewarning, if you win, that within the first ten minutes, she'll bring up how you shouldn't have spent that much money to go on a date with her.