As to where to add returns, the goal is to return the air that is supplied to each room, so I doubt adding return to basement is the way to go.
Definately run additional returns back to the furnace.
Though many refuse to believe it, it has been proven that the location of the return grille, high or low ,in the room has little effect on comfort.so do what is easy to do.
Not sure why they are adding returns, though I don't doubt they are needed. it is usually due to rooms not being heated enough, or high static pressure ,which is resistance of the duct system to air flow.
When static is high, they should consider adding "turning vanes" to the elbows",90 degree turns" , in the main ducts. This is a simple way to reduce static, that most contractors fail to utilize. Ask about turning vanes, positive response, gets the job!
We replace around 750 systems per year , and 50% plus need added retuns ,due to high static. High static, will not let the equipment deliver the effieceny savings that it is capable of. So yes, it has been costing too much to operate since the day it was installed.