The NCL package also includes $10 in Universal Dollars. Lunch Money. So if you do it yourself you are at $352 plus transportation. Rental car and gas and parking $ 80 ?
Now you are at $432 or about a $150 premium. The question to ask is it worth it to be sure you get the express pass. The summer is busy and spring cruisers were shut out. I did the NCL express package in July last year(only 2 of us) and would not want to go without it for the short time you have. Also you have to alow extra time to get from the pier to the rental car company and back to the ship at the end of the day. On the NCL package the ship will wait on your own it's a long drive to Miami.
It is all about buget vs. risk. Sort of like the travel insurance debate. There is a different correct answer for every situation.