Hi MDIS.
I had done similar research to yourself. However, as there is a range of prices I didn't want to publish figures for fear of being accused of bias. Nevertheless, I did conclude that it was significantly cheaper to fly from other airports. You rightly cost in the price of car-parking. However, you should do this for Manston too, because most of the intended market does not live within walking/taxi tange of the airport. At £25 per day, that's £175 a week. For a person living in Chatham they have a choice of driving to Manston or Gatwick. It will take them about the same time. I suspect the overall cost will play a big mart in their decision of which airport to use. Gatwick comes out much cheaper overall.