The 4 cylinder is gone, so the 6 is the only relevent number unless you want a used car.
The 2011, 4 cylinder CX had a base price of $31645, the 2012 eAssist has a base of $35415, for a difference of $3770. I don't know if standard equipment is the same for each, and there was a price increase going from year to year, so it may be apples to oranges.
Having said that, using the default numbers from the EPA website, but current Canadian gas prices ($1.19/L, $4.50/gal), you'd be saving $607 a year, or about 6 years to recoupe the price difference between the 2011 4 cylinder and the 2012 eAssist.
If, like me, most of your driving is city (80%) the payback is just a bit over a year.
Price changes year-to-year aren't dramatic so the basic comparison still stands.
More questions. If you were to put a price tag on the other features that suffered due to the addition of the eAss-
ist nicht gut like hampered trunk space, plus the added complexity that, given GM track record, is bound to give many an owner major headaches once the warranty's out.
Plus, you have to put a lot of stock in EPA numbers to make that claim that the price difference can be recouped in the timeframe you indicate.
Too many variables and none in GM's favour.
If you're after comfort and fuel economy, go for the VW Passat 2.0T diesel. Much better fuel economy guaranteed, more passenger and cargo space, better torque, better warranty, and much CHEAPER to boot. Passat is 'premium enough' to be cross shopped against the Regal given their brand value, drivetrain options, and amenities.