Zines: How do they work?

I have had this problem for a week, and it's only now that I've realised that I probably know a bunch of people who know the answer to this question.

How much should one pay for a pay-what-you-want pdf zine?

I know the answer is technically 'whatever you want!' but I have no clue what a reasonable amount is. Is there a formula of pages to dollars, multiply it by this number if it's in colour, or something like that?