I have a retired Jeweler friend that plated my casings for "trade"
WhenI inquired as to the $ amount, he said it should go for about $5.00.
He doesn't do plating often,it was a rare occurance that he had to set the equipment up and remembered I had asked him about plating some stuff for me.
Regardless the cost I quoted I would check around, even if it were doubled or tripled that would make the item more valuable to the customer,after all they requested it.( and are paying for it)
I had enough plated to speculate,it would be tougher if it were a "custom" request.