One note: I think the fares are $44.15 for a car and driver, and you have to pay $13.45 for each of the 3 passengers -- so the car fare is $84.50 vs. $69.80 for 4 cyclists (including their bikes).
After taking ferries around the islands this weekend, I'd pay a premium *not* to have a car. As noted by woodway, it can be hard to get a car off the islands on the weekend. You have to sit in line to get your car out, and some people end up waiting through several ferries before they can get off the island.