The OP comes from Australia. A RT ticket may not be the cheapest option. I personally would (and have in the past) buy a one-way to the US and a FULLY REFUNDABLE one-way to somewhere outside the US with a departure date well after your arrival, but well within your allowed stay (three month - I assume you come on a visa waiver).
You could book for instance this trip LAX-MEX
Lufthansa - Reservation and booking
(you must click the "Click for flexible conditions" checkbox)
It comes with the following fare rules:
The fare that you have selected is fully flexible and can be rebooked and cancelled free of charge.
This fare is only valid if the flights are taken in the booked sequence. Otherwise the fare will be recalculated based on the actual flight routing.
For further Information about the fare conditions please call the Lufthansa Service Center in Germany at +49 - 1805 - 838 055 (e.g. EUR 0.14/min. from the Deutsche Telekom landline network; call charges may vary dependent upon network operator or mobile network provider).
To find out the telephone number of the Lufthansa Service Center in your country please click here.
So, you pay for your flight ($1000 in this example). Once you are inside the US (do not mention your motorbike being shipped, keep saying that you are a tourist) you simply cancel your flight and get the full refund. With Lufthansa this will take up to 4 weeks. So you do need to have the money as your credit card bill is generally due before you get the refund.
If you have the spare cash, all you lose is a few bucks in missed interest. The ticket is so ridiculously expensive because it can be changed and cancelled at no cost.
The key is to buy DIRECTLY from a REPUTABLE airline and to get a FULLY REFUNDABLE fare. Check the fare rules!
If you do want to chance it, make sure you have enough money on your card. I can almost guarantee you that you will be forced to buy a ticket out of the US at checkin in Australia. If they do, keep in mind that you do not have to buy a ticket from the airline you are flying with. Just step back from the counter, get on the net and buy a fully refundable one-way out of the US as per example above and be prepared to pay quite a bit more as you are closer to your (non-)flight date.
The other option, if you don't have a $1000 to spare temporarily, is to buy the cheapest one-way out (not including Canada) and eat up the loss. This one is obviously harder to do when you get turned down at checkin. Cheap flights may not be available at that point.