A conservative think tank estimated Monday that granting citizenship to 11 million people in the U.S. illegally would cost taxpayers trillions of dollars, a conclusion that split the Republican Party and added a new level of complexity to efforts in the Senate to overhaul immigration laws.

The analysis from the Heritage Foundation concluded that the net cost would be at least $6.3 trillion in inflation-adjusted terms as immigrants gaining legal status would receive $9.4 trillion in government benefits and services over their...