In the small scale, producing infrastructure that can bill your meter going both ways only increases the cost.
But in the large scale, not having to build and maintain more high-power distribution towers across hundreds of miles of difficult to access terrain decreases cost.
You can't plop down a coal plant in a city, but you still have some freedom to choose its location, and you can scale it to the needs of the local area and just burn as much as you need. Meanwhile, solar and wind might go way down for a whole day or more, and so you need to ship energy from where it's being generated.
At some point those cars can actually be part of the grid's storage capacity, if the infrastructure is built to allow the car to put some of it's capacity back onto the grid during low availability. EV drivers willing to have 20% of their battery be drained when they need to drive can get paid a small amount for leaving their vehicle plugged in while not in use.