If your sales are falling you have to go back to what made them rise in the first place. In Samsung case copying Apple. The surprise will be Xioami giving then a taste of their own medicine by copying Samsung like you said. Karma.
Good estimate. I made the same analysis when Apple announced the watch and reached almost the same number. The only unknowns are cost of the leather strap and the sapphire glass since the sport Apple watch have different strap and glass. The leather might not be that expensive but any idea how much sapphire on other watches cost?