I have been taking a trike for some years originally a Canam two wheels at the front and now a Harley triglide. The Steam Packet calculate the fare as the same as a motorcycle and sidecar so you can book on line.
My fare from memory was about £240 this year. The only problem is they load you after all the bikes are on so often a long wait irrespective of what time you arrive. Look out for a white Harley Triglide when you get to the island.