Response from Ryan:
Hi Ben, you weren't overcharged as we have no way of increasing the price after you book - Airbnb is pretty strict about these things.
You didn't book just a room, you booked the entire apartment, for 4 people, over the weekend, in the middle of St Kilda, for 2 nights, with early check in and late check out. And it cost you around $44 per person per night.
If you issue is with not knowing how to read the price before you book, I suggest you should contact the host / Airbnb next and and confirm what will be the total cost first.
Alternatively, if you enter the dates you booked along with the number of people you will get the price quoted. Thanks.