Prix libre / open pricing / free pricing

Hi !
I don’t know how you express this in english; but in french it’s “prix libre” : when a customer can choose the price for something.

Would it be possible to add a way to have “prix libres” in The Events Calendar ?
It could be, for example; when the price value is -1 (since 0 is used when it’s free with your plugin).

Thanks !!