AA Membership website
Try booking through AA if you are a member. I've used it for both Gatwick and Stansted and been happy with the result. They give you a range of options and it's up to you to balance price and convenience.
Do agree thouigh that this is a racket. Even dropping off a friend or family member costs unless you leave them at a remote car park to take the bus in. And if there is a queue to exit at the terminal, the cost is jaw-dropping.