Are you making just 1? Not so much.
I try and make 10 and 20 cards at a time to keep shipping costs/lot down and take advantage of lower costs on bulk buys.
And I don't have the overhead costs of the big manufacturers to carry, its just me & MrsCrossroads in a workroom upstairs. Toaster oven in the kitchen on occasion. Table in the living holding kits & assembled boards waiting to be packaged & delivered.
For "average retail" I've heard that retailers sell for twice what it costs them to purchase from wholesalers to cover material, fee, overhead, salaries, insurance, rent, utilities, etc. I don't know if low level electronics retailers work the same. That's the best I'll hazard for a guess.