Actually, only a small percentage of Puerto Rico's pay US federal income tax. Income earned on the island is essentially exempt unless it's from working for the US government. Here's an IRS note on the topic:
"If you're a bona fide resident of Puerto Rico during the entire tax year, you generally aren't required to file a U.S. federal income tax return..."
I'm having a hard time finding hard data, but the last number I could find, from 2010, showed the island paying $20 million total in individual federal income taxes, on a population of 3.7 million.