There's no charge to park at resorts. Commonly they'll give you a three-hour parking pass so you can eat or just check out the resort. If things are very slow they may not give you a time limit. If things are incredibly busy, they sometimes close the resort parking lot for everyone who's not a guest, but that's rare.
One thing to know is that if you pay for valet parking once at one resort, it's good for valet parking at any resort for the whole day. So if you spend a day resort hopping, you can valet park everywhere for one price. You still should tip the valets at each place, but that's just a couple bucks or so. Also, if you valet park you have no time limit, and I would guess you'll never be locked out during busy times.