I’m not an old Monza hand by any stretch, but I was there last year.
We were sitting at Parabolica but parked north of the circuit (train your eyes left of Curva Grande on google maps and you’ll see two soccer pitches, that was the carpark). We arrived at about 9am and I think it cost €20 for parking. There wasn’t much traffic so we drove straight into the car park (which we just stumbled upon really) without any problems.
I too wanted to make a quick exit as I had to be back in Germany for work the next day. With about 5 laps to go we crossed to the other side of the track and made for the exit gate that would take us onto the track for the podium celebrations (got a pic of Alonso with the camera!). Afterwards we walked down to turn 1 for a look around and to soak it all up. From there we headed back to the car and unbelievably there was ZERO traffic as we left, we were on the road back straight away.
So an option for you might be to sit in the grandstand at the outside of turn 1, which is probably one of the most expensive but has one of the best views. From there you could be on your way 5 minutes after the chequered flag if you want!