Where I worked the maids were paid much more than the drivers. Maybe things have changed in the years since - but drivers were expected to get tipped, therefore they paid us much less. Not all people tipped - but the airline crews *always* tipped - $3.50 per crew.
Alot people on here are talking about negotiated rates and such by the hotel, I don't know - that may be true - but the driver isn't getting any of that. I still got below minimum wage whether it was negotiated or not. Also - some are mentioning that they give a tip only if the driver does more than pick them up and takes to the hotel/airport. I worked at the Sheraton Century Center (downtown OKC) and it was against the hotel rules for me to make side trips for food and such - probably some legality thing - but in essence when I took the crews out to eat, or stopped enroute, I was risking my job.

USMCFLYR