Adding to that, Jose Iglesias was signed at 19 years old for 4 years/$8.25 million. He was considered a budding superstar in Cuba at the time, as well.
When you look at the history of Cuban signings, most of them don't look too bad. The trick, like with all Latin American players, is not getting fooled by falsified documents, and those issues seem to be getting cleaned up more and more each year, especially with these Cuban defectors.
I would guess with Diaz's age and his relatively low profile, he wouldn't cost much more than $3-4 million AAV for four years.