Prestige to Dream is a significant upgrade. The approximate cost difference can be found by just pricing out a test rental on Hertz.com, but of course, everything is negotiable at the rental counter (depending on availability). Being President's Circle doesn't really help in concrete terms, but given your status, the agent may be willing to be a little more negotiable. Don't hesitate to nicely point out that you are PC, and if you're not hearing a rate that is acceptable, ask to speak with a manager to see if they are willing to do any better. Certainly nothing to lose.