Further to the answer above, "at the door" also implies a little bit of risk, e.g. if you're lucky, you'll be able to buy a ticket at the door (when you arrive) if the event is not already sold out.

Hi! At the door here means : if you come to buy a ticket in person , it would cost so and so, as opposed to online which costs a little more, due to involvement of taxes, etc. Thanks Syed