Sorry this is happening to you, Allanah and you are correct, the rate at the time of booking cannot be changed. I as a host have made the same mistake in not updating my rates for special events and had to just deal with the loss. Never would I burden the guest with my error. Air BNB will back you up and if the host cancels you, there are many penalties for doing so.
Now, all that being said, do you still want to stay with your reservation? Would you feel uncomfortable there? If you think so, ask for Air BNB help to find a comparable place at the same or lower cost. Some people have good response by using Twitter to contact Air BNB rather than wait on the phone for a long time.
Good luck!