Undoubtably the cost would go up, but nobody is building out datacenters full of DDR4, either, so I don't figure it would go up nearly as much as DDR5 is right now.
https://pcpartpicker.com/trends/price/memory/
You can see the cost rise of DDR4 here.
https://pcpartpicker.com/trends/price/memory/
You can see the cost rise of DDR4 here.