It's $6 per day. You can move the car to another park without paying again. If you have an annual pass, you get free parking no matter where you stay, so it maybe worth it for one person in your party to buy one.
If you are visiting a park just drive to the parks parking lot. It is $6 per day unless you have an AP. Keep your receipt because if you hop from the MK in the morning to EC at night you will not have to pay another $6 to park at the EC lot.