How flat does a subfloor need to be for laminate?
Check the specifications for the particular laminate you are planning to install.  For example, Armstrong Flooring specify:


  Variations in subfloor flatness should not exceed 3/16" in 10' (4.76
  mm in 3.05 m) or 1/8" in 6' (3.17 mm in 1.83 m). Level floors with a
  suitable cement-based self-leveling underlayment following the
  manufacturer’s recommended guidelines.


If you do have to level the floor, you are going to need a lot of fill.  You might want to use something other than self-levelling compound.