Sorry for my late reply. I'm looking into where I can buy long boxes, but if I ship through USPS it shouldn't be more than $20, for FedEx I have no clue! I'll have an answer for you in 25 minutes.
Is this the same guitar that was up for auction before? I hate it when an auction goes south. Well, I would if it'd have happened to me. If this has a bolt on neck, you might be able to separate the neck from the body, after removing the strings of course. That would make shipping a bunch easier. Good luck with the auction.
It's not a real guitar, but I'm not going to charge more that $15 I'm shopping for boxes tomorrow, I will find the cheapest shipping, and it WILL be as cheap as possible!