You are correct that a portion of your party cost goes to the driver. I don’t know if that necessarily means the cost is higher, but I’ve read other posts indicating that using the limo actually lowers the amount your lady actually receives. From what I’ve read, the driver gets a percentage of the party cost. That’s why other posts suggest driving yourself there so the lady gets more (in my case I didn’t have a choice but I’m going to definitely drive there for my next trip).
IMO, you would tip the driver for the same reason you would tip a waiter, a cab driver, etc. Just think about the time he has to put in. About 4 hours of drive time (it’s about 1 hour from LV to the ranch) plus a couple of hours while your party is going on. Assuming he doesn’t drive to/from LV while your party is going on, that’s around 6 hours. It’s like any other tip that you just give him whatever you feel is appropriate for the service your received.