More advice: get a cheaper PCB supplier. My largest board, the MIRE, costs less than $6 including delivery from PCBway, with immersion gold. It's about 7" x 6.5".
Just realized you meant the daughter board. Yeah those can be expensive. The Ethernet on MISE/M3SE runs $20 and the FPGA module around $15.
Yes, I do prototypes with OshPark - they are expensive but use real coppy. You get 3 PCBs ~70 $. At the same price I am getting about 50 from Seeed - they are ok too, but tin only.
Using OshPark I wouldn't be able to sell it for 80.80 $. I have sold the extra OshPark PCBs as well, but at + 10 $. I leave it up to the builders which one to use. Cheap and China and Tin, or Expansive and USA and Copper.