I once called Amex Platinum Concierge to ask about whether they could give me a deal on car service. The very honest gentleman at Amex said "Honestly, Sir, I don't recommend going through us if price is a concern." He went on to tell me that they use the most expensive services, and generally at a kickback-type rate.

When not using public transportation or arranging rides from friends/acquaintances (I hate airport parking, and I hate shuttle vans), I have had good luck negotiating with underworked cab drivers to provide meet-and-greet type service. Only works if you don't mind riding in a crappy cab (I don't). In my experience, dedicated sedan companies are overpriced and provide poor service -- the drivers have no stake in what they're doing (and have no economic reason to care).