The cost depends on the length of your vehicle. When we took the ferry on our RV trip in 2013, I took the spare off the back of the trailer and locked it in the bed of my truck just to knock 18" off the length and reduce my overall cost.
Too bad that the Alaska legislature can't afford to replace the Columbia with a new ferry. It's not the oldest ferry on the AMHS, but it will be 44 years old next summer and has a lot of miles on its machinery. In 2000, it had an electrical fire that put it out of commission a week before we were scheduled to take it from Sitka to Petersburg and Wrangell. It was towed into Juneau about the time we flew from Sitka back to Juneau using our ferry refund for the airfares.