We use the summer special car park at Gatwick, it's near the N terminal but it runs a shuttle bus every 20 mins to both terminals. Under £9 a day, but we book via Holiday Extras website and they have good deals from time to time. You can get it a bit cheaper on further-away car parks but I worry about transit times, especially as we usually travel early in the morning. Summer Special has been around for several years and we have had no complaints for the many times we've parked there.