He isn't promising a 150% ROI by the technical definition of ROI. That would mean he would pay you 150% per year. He is promising you pay you back, plus 50% and that would be the end of it. And he doesn't promise a time frame either. Personally getting 50% of my money back in an undisclosed time frame doesn't really encourage me to invest, I would expect 100% on a risky proposition like this.