Event Price Sets do not permit Prices of 0.

Price sets work to store and display a collection of event signup choices, like "Gold Pass", "$100.0".

One of the choices I want to display on the registration form is "Drop-in" or "Inquiry". I don't want to charge these people but I do want to capture their interest and their registration (or pre-registration) in the same registration form. In my use case it doesn't matter whether they are then set to "pay later" or just not charged.