At high speed Porsche recommends having the soft top up, because it improves the aerodynamics of the car. So the hardtop makes the car more aerodynamic. They only weigh about 55-60lbs. so they don't add to much weight. They add no safety, and the prices range from $600 (if you are very lucky) to $2000 used, and $3000 new. I got mine for $900 and by freak chance in the same color, the average asking price is $2000 but most people pay $1500.
