I believe there is an excellent chance that TDKR will gross more than the Dark Knight. Often, the third movie of a trilogy grosses more than the second movie:
Spider Man II: Worldwide $783,766,341
Spider Man III: Worldwide: $890,871,626
X men II: Worldwide $407,711,549
X men III: Worldwide $459,359,555
The Lord of the Rings: The Two Towers: Worldwide: $925,282,504
The Lord of the Rings: The Return of the King: Worldwide: $1,119,110,941
The Bourne Supremacy: Worldwide $288,500,217
The Bourne ultimatum: Worldwide: $442,824,138
On the other hand, in these trilogies, the third movie did not make more: Matrix, Pirates of the Caribbean, Rush Hour, Mission Impossible, Blade, and others.
There seems to be a rough correlation between the overall quality of the second film, and the overall gross of the third. There is no glaring examples better than Spider Man and X Men. In the trilogies that have a poor second film, the third film tends not to gross more.