If she wants to believe, why not help her?
My daughter is getting a little skeptical but she absolutely wants to believe, so I help her do that. If she asks, I say, "Well... what do you think? Do you really think I'd be the one buying all these toys?" etc. She knows I'm cheap and tight, so that helps.
If she was genuinely wanting to know, I'd tell teh truth, but she WANTS to believe, she is clinging to it hoping it's real so I'll help her believe it if I can for the next few years.