Once you figure out what (or if) you'd pay for Express Pass, then look at the prices for staying at one of the top three hotels at Universal. (Hard Rock, Portofino or Royal Pacific) And compare that to Express Passes on top of the cost of wherever you're staying. If you stay in those three hotels everyone in your room gets Unlimited Express for every day of your stay, including checkin and checkout days.
When we went in November we were staying 7-nights at a timeshare in the area, but added on one night at Royal Pacific at the start. We had an early morning flight that got us into Orlando around 10. Picked up the rental car and we were at the hotel by 11, and on our way to the parks by 12:30. Between that day and the next (a Saturday and a Sunday), we Express Passed almost every attraction at least once. It allowed us to make sure we got to ride the headliners, and then tour the parks more leisurely when we returned later in the week without Express Pass.
Of course you should also check out the expected crowd calendar for the time you're going. We're heading down this Sunday for an impromptu 3-night visit (those annual passes really are hard to resist). We'll be at Cabana Bay so no Express Passes, but the crowds in the parks are 2's and 3's according to Touring Plans, so we're not really worried about lines.
-Rob
