He simply realized that the markup on drug prices was so mind-bogglingly absurd (oftentimes over 2500% markup) that he could undercut the market by thousands of dollars and still make a tidy 15% profit.

When the libertarians tell you fan fiction about the “invisible hand of the free market,” this is what they mean. And the fact that it took this long for someone like Cuban to come along should tell you how viable a strategy it is.