Elon Musk's big purchase is a powerful signal. But investors still need to weigh what they're paying for.Tesla (TSLA 3.54%), the electric-vehicle maker that's rapidly positioning itself as an artificial intelligence (AI) and robotics company, jumped after CEO Elon Musk disclosed a large insider purchase. The move -- his first open-market buy in years -- reignited debate about whether the stock is attractive today or still priced for perfection.The buy matters. Signals from founders and CEOs can be useful co ...