Attendees should have an amount_owed field like Groups
Both attendee and group registrations have a dynamically calculated "how much does this registration cost" amount. However, group registrations have the option to manually override this, to apply discounts or extra charges for upgrading a registration, etc. This allows us to dynamically generate Paypal buttons with the correct amount which are tied to the group.
This should be implemented for Attendees as well, as it will allow us to e.g. upgrade people's registrations, etc.