You only have to pay for ECV one time. Just keep your receipt. Remember that there are a limited number of ECV's available. So if you are going on a crowded day or late in the day at a certain park there might not be any left. There are also outside vendors you can rent from. There is a lot of information about ECV's in disablites sticky thread
You can't take the ECV rented in the park out of that park.
As was mentioned, you will be able to show your receipt and get an ECV in the park you are hopping to if they have any ECVs left when you arrive there.