I used to take the Greenway in and Rt. 7 back to save on the tolls.
But a major difficulty is that the Greenway is not and never has been profitable -- and VDOT knew it wouldn't be, which is why they didn't extend the Dulles Toll Road out past the airport to begin with. As I understand it it was influential people in Loudoun who exerted enough political muscle to get it authorized as a private toll road, and even then it was with the stipulation that VDOT would not spend any resources on it, it would have to pay for police patrol activity and eminent domain could not be used to acquire land for it.
Given the lack of profitability and that the road is privately owned, any pressure from the state to reduce tolls might just result in the Greenway's total abandonment by its owners. Then VDOT has to either close it, or take over the maintenance of it.
"If ye love wealth greater than liberty, the tranquility of servitude greater than the animating contest for freedom, go home from us in peace. We seek not your counsel, nor your arms. Crouch down and lick the hand that feeds you; May your chains set lightly upon you, and may posterity forget that ye were our countrymen." -- Samuel Adams