Last week it appeared that Martinez had reached a deal with the San Francisco Giants for a reported $2,500,000. However, MLB ruled that the reported agreement with the Giants was not binding; Martinez apparently had multiple representatives simultaneously dealing with various teams and the Cubs deal was preferred.
Martinez is a 20-year-old right-handed hitter, listed at 6-0, 190. Ben Badler's report for Baseball America indicates that Martinez has solid-average tools in most respects along with a quality line drive bat, but was not considered among the most elite players on the island. Early rumors that Martinez could command a bonus of nearly $10,000,000 proved unrealistic once teams got a better look at him.
$3,000,000 is still a large amount of money, equivalent to a mid first-round draft choice. The Cubs are well past their international signing bonus pool for 2015 so they will pay a 100% overage tax on Martinez's bonus.
Here's some video of Martinez from Baseball America.