Hi Dee,
My wife and I stayed in Dymchurch in June and we had two daytrips across to Calais. The car parks at Dover terminal are quite expensive - �12 per day I think. The car parks in the town are about �6 per day.
We didn't want to pay those prices so we parked at the bottom of Castle Hill which is a nearby side street leading up to the castle. It cost us nothing to park there all day. Perhaps the residents weren't happy but it was on week days and there's no reason not to park there. There are several other side streets and roads where you can park free if you have a drive around.
Castle Hill is only about five minutes walk for us to the terminal and it wouldn't be much more for a couple of wheelchairs. If you have friends who are not so mobile then it may be best to pay the �12 in the terminal car park. Hope that helps.