It depends on how far across the country, but try and keep the box under 51" long and 28" high & 8" wide. if you go over that, the shipping almost doubles.. getting it apart may be the hard part. depending on how bad the rust is. but cost should be around 80.00 to ship across the country.
You can also find some decent youtube vids on how to properly breakdown and pack a bike for shipping.
You can pick up a bike box from you local bike shop (LBS) for free. Some LBS's will even do the breakdown and packing for you for $40-50.