I would GLADLY pay an add on for parking, even if it is a bit steep. I just loathe the idea of paying nearly $20 every single time to get in. I HATE that. Even Disney, the "stingiest" of the market, offers free parking to premium holders.
I don't think this will last. Even with Potter, I think they will offer some sort of parking add-on. People are not going to take this very well.