Well in IV if you didn't have enough money it would just throw you out up to the point you could afford, but in V it's better because it's more like real life. The driver would call the cops at the end if you didn't pay.
You don't pay the driver when you get out, you pay the driver when you get to your location. You have to wait until he pulls up or you'll get a wanted level. Just skip to the destination if you want to be sure.
You also get a wanted level if you don't have enough money to pay, but given I amassed nearly 2 billion from the stock markets, that won't happen for me any time soon.