While looking into HDI – High Density Interconnects I found https://pcb4u.com/Quotes/Board?cpn=3 Accutrace Inc. offering a 20sq in PCB for $10 2 layer, $15 4 layer with $10 FedEx.
Current 3D printer controllers are 110 X 85mm or 14.4 sq in. with 20 sq in you could add 6 more stepper motor driver!
Has anyone tried https://pcb4u.com?