JLCPCB should still have special offers for 4-layer PCBs up to 50mm tall and wide. It might be that you can get five boards made for less than 10€/$. I’ll check later.

Edit: yes, $2 for five pieces of 6-layer, 1.2mm thickness PCBs with ENIG surface finish. Unbelievable, isn’t it? 4-layer, 0.8mm, are possible for a few dollars more if thickness matters a lot (only leadfree HASL, though).