I kinda think it depends on how long the drive is, what they do for you, etc... In Vegas, a limo from one property to another was worth $10 to me -- from the airport, $20. A pickup from the airport to Harrah's in New Orleans is quite the drive -- we tipped $30 each time.
Some drivers are very helpful with your bags, or will stop at a store and let you get drinks to take with you, etc... That obviously should be factored into their tip.