After adding the RS485 communication circuit, I logged in JLC online system, placed an order for PCBs as well as SMT solder to produce 30 piece samples.

Total cost for PCB + SMT is 668.51RMB, 22.28 per piece;
Plus 6.60RMB for the ESP8266-07S module, 2RMB for the terminals, 6RMB for the housing, and 3RMB for the relay;

Total cost is 40RMB;

if the selling price could be 50RMB excluding freight, I won't lose money.

Need continue to investigate how to reduce cost, the current production cost per piece of about 13 RMB, I think the production cost can be reduced steeply in mass production.