The cost is probably not that bad on a per-unit basis, a 50mL seems like it should last for a pretty large number of PCBs. The problem is that the resulting PCBs are almost completely useless because you can't solder anything to them without melting the substrate.

The ink that works with standard inkjet printers just doesn't stick to materials like FR4 or kapton. There are specialized machines for printing conductive ink onto usable PCB substrates either with an extruder or screenprinting but they are at a minimum several thousand dollars and have extremely expensive consumables.