How much energy is wasted by plugging a laptop wall charger into an automotive inverter
There is no way to know about your case without specific knowledge of your inverter.  You therefore need to look up the efficiency of your particular inverter at that particular operating point, or measure it.  A 700 W inverter may be reasonably efficient at 650 W, but could be horribly inefficient at 50 W, for example.

Actually, you should measure the overall efficiency from car battery to DC output of the charger, then compare that to a similar setup that works directly from the car battery.  That's the only way to know for sure.