I agree with Maartin, it costs no more to ship by air than it does by sea, and it is a heck of a lot simpler to ship by air.
The 'base price' by sea might appear to be fractionally cheaper, but once you add in all the port charges, it's the same or more than air.
The trick seems to be to find a charter airline that uses wide body aircraft (e.g. L-1011, DC-10, MD-11, B-747 or similar). The charter carriers ususally fill the main deck with passengers who are limited to two bags each. This often leaves a great deal of cubic space available on the lower deck.
A motorcycle inside a container (typically a LD-6) takes up a lot of space, but does not weigh very much, relative to normal air cargo. For example, a 350 kg motorcycle is equal to the baggage checked by only 6 or 8 passengers. The charter carriers will often carry motorcycles on a 'space available' basis for very attractive rates (e.g. less than US$ 1,000 each way across the Atlantic).
You could shop yourself, or contact a consolidator, or contact a company that specializes in motorcycle shipping. I have used Motorcycle Express (www.motorcycleexpress.com
) and was very pleased with their prices and their service.