@stal2walk The manga raws say white gold coins so yes, it's just a mistranslation/lazy translation.
Also a quick FYI, One platinum coin = 10 gold coins = 10 million yen = 90,000 USD (or 100k rounded). So the king basically gave him 1 million USD. Don't remember if this conversion was mentioned in the manga.
@WhimsiCat @maffo666 The 10% is indeed sales tax, but the last 20% isn't exactly profit.
The actual breakdown should be:
"Ideas portion/fee" (you can think of it as royalties, although it's technically more nuanced than that): 20%
Sales Tax: 10%
Cost of materials: 30%
Shipping and Transportation costs: 20%
Company operations cost: 20%
So that last 20% isn't exactly straight profit for the company, since that 20% portion would include stuff like the cost of labor and overhead. This means the total "Production Cost" of the boards would be 50%. I assume the "company profits" would have to be cut out of 20% operation costs; or the company doesn't really make a profit at all (workers would still get paid with operation costs of course).
PS: I give the TL a slight pass on the percentages mistranslation since they are hard to read, but omitting the white gold/platinum coin part is just laziness.