Be aware, Dakar Motos doesn't appreciate it when they find out that you have shopped around. Or at least, they mentioned that to us when we used their services and were not amused by it. :-S. As you mention, not everybody is positive...
For flight, you might try to ship from other countries, Argentina seems not very easy.
For container, contact 'Smelly Biker', you'll find him here or on adv-rider. I never used his services, but asked for a few quotes and he seemed to be a very nice and reasonable guy.