Hi,
wanted also to go from Santa Cruz to Corumba (brasil). We took the train in the end and had no problems at all. Ok, it was shaking the whole night and once the train left for a short time its track (great adventure), but in the end it was great fun. I recommend to use a insect repellant!
Oh, a friend of mine from Sao Paulo did the road along the traintracks and it took him 3 days until Corumba (there are some sandy areas). So thats about all.
In case you need the document "temporario imporation del vehiculo" ask for it at the border bureau of Corumba (very slow officers, but they can do it. It took them about 2 hours for the 2 documents) and to get the stamp in your passport go to the Rodoviaria in Corumba city (ask Motorbike taxis for the right way). As long as your motorbike is not registered in South america do not care about the speed limit controll points (with Radar). In Brasil it has a lot of them, but they will never send you a bill to an other continent.

Have a nice ride.
PS: In case you go by train, you will probably need a reservation for the space in the train for the bikes (we did that right next to the train station)

Sorry forgot something:
regarding the costs:
should be about 0.6 Bolivianos /kg and we paid about 40 Bolivianos for loading and 30 Bolivianos for unloading
the policeman at the loading in Santa Cruz is very friendly and will help you, if you ask him.
Take material with you for good and safe installation of the bike in the train. The train will be shaking a lot!!!! and the whole time!!!!