How do I go about setting a price for a small program?

Having worked for a large company for ten years I’ve never sought to freelance. An opportunity has presented itself and when the customer asked how much it would cost, I stumbled like an idiot and said I didn’t know. She’s called back twice now asking if I can do it, and how much it will cost. She’s also mentioned that she has a lot of affiliates and sponsors who are in need of such services (someone to create small programs, using Access or Lotus Notes). Please advise – I’m as ‘green’ as they come!
Thank you.