Charging for the thing I already give away free

I have a free five-part email series that a lot of people finish, and I keep getting asked whether there's a paid version. I've been putting it off because I don't have anything that proves it's worth money — no testimonials, no completion numbers I trust. But I also can't keep answering the same questions one-to-one. Before I build anything, I want to know what actually made people pay.