Was he? Why do you think we're all smarter than him? He may have well know that he was being scammed and used the hit thing as a bargaining chip to pay only $150K instead of $500K. It's a very considerable difference, after all. Many of you would kill just to get your hands on those $350K he saved 
I thought that too. The problem with that reasoning is that he'd be trusting the blackmailer not to attempt to blackmail him again in the future.
There's also the possibility of this whole hired murder thing being fake. The US gov is definitely not a trustworthy entity. Maybe they preferred to create this story so that DPR's image is severely tainted, and he cannot present himself in courts as a "peaceful individual" who was "removing violence from drug trade".
All this is just speculation, though.