I am probably off on the $2 a foot price. I occasionally install Hardwood and more often hire it out; $2.50 - 3.00 a foot is pretty typical for hardwood around here. Laminate doesnt require sorting and obviously no nailing so $2 a foot is probably still high. Ive never have contracted for laminate though I have done small installations and repairs. I do have 800ft of laminate tile to put in the basement of our rental property and I plan to spend no more than a day on the install so, yes I suppose $2 a foot would be a bit much!
More on the leveling:
I use the string method when I am concerned about major deflection in a floor, taking sag out of a beam for instance. It will show major high and low areas over larger lengths. It may be helpful in deciding exactly where to start.
The 4’ straight edge as Hampshire suggests is certainly all that is required to find surface issues that are going to effect the stability of the laminate. Even a piece of the laminate itself is going to be very effective at showing what it can bridge and what it cant. I do use this method (with only a 2’straight edge) for hardwood on wood subfloors when swelled joints are an issue, and take down offending areas with a belt sander.
A basement slab is a different story as taking down high points is a major task and leveling compound becomes very expensive if used for a large area. Just get it good enough so the laminate isn’t going to creak or move.