Not negligible, but also a small cost of the total cost of the iPhone system. call it 500 megawatts for 100m iPhones, which for moderately expensive generation capacity is ~ 2million per megawatt of capacity, works out to $10 per phone in electricity. A typical power plant has expected lifetimes of ~25-50 years, which if we assume a generous 5 year phone lifespan, and a 25 year plant life...

That works out to $2 per iPhone to build power plants to charge the iphone every day for it's an entire life. Not bad for a device that costs hundreds of dollars.