@bushwhacker2k looking at the source, I believe it was mistranslated as "White Gold" in earlier chapters and should always have been "Platinum"
Then again, I also believe the Mangaka made a mistake when originally writing the ring costs
The page actually says "3 Million Cents"in Japanese which the scanlation team converted to "3 Platinum" based on previous chapters
But then the MC calls this 3 Billion Yen (both in Japanese and in the scanlation), when in fact based on chapters 3, 5, 10, 11, and 16 (again reading the original Japanese) should be 300 Million Yen (Ch5 50"White Gold"/Platinum =5 Billion Yen, other chapters back up this calculation)
So, I think from Ch10 on it is easier just to think in terms of "Platinum" since CH5 has translation errors in the amounts and the Mangaka appears to be inconsistent in the isekei to yen translations in this chapter