Space based solar power expert John Mankins, the author of the book The Case for Space Solar Power, estimates that if the launch price came down to about $400 per kilogram then space-based solar power would be cost-competitive with terrestrial solar energy.
A SpaceX Falcon Heavy will launch 63800 kilograms into low earth orbit for $90 million. This will be $1411 per kilogram.
SpaceX should be able to land the first stages for reuse. If they can get to a little over the ten reuses planned for the Falcon 9 block 5 then they could bring the costs down to $743 per kilogram.
This would bring the Mankin space-based solar power cost down to about 3 to 5 cents per kilowatt hour.