IIRC, the E-190 is cheaper than the A-318, at least that is what I was told. It probably helps being launch customer as well.
The A-318 is relatively heavy, since it is essentially a shrunken A-320, but without the resultant weightloss. Hence fuelburn is higher as is cost to operate, plus the engine differs from the A-319/320.
190 318
Max take off 105 130
Empty 62 86
Ldg 95 123
All numbers in pounds x 1000
It is disappointing to see the early results of the 190, would have thought, that since it is a stretched 170, that most of the quirks and gremlins would have been worked out. It appears not, however, I have little doubt, despite these early problems, that the airplane will work out for its intended mission.