I believe Platinum is one of the highly expensive commodities (more than Gold and even more than diamonds in cases of the types of quality offered as platinum is highly rarely in findings). Though, it's better not to compare Bitcoin to these things as it's not yet a commodity but a speculative asset. Still if you would like to compare, Bitcoin is way beyond being called as expensive but in my views, though it has got a very handsome price tag attached, it is still far from being the number 1 in terms of market cap. Bitcoin needs to be very, very extraordinarily expensive from anybody's reach (even a rich person) to be able to get to a market cap so competitive that Gold would be like 1/10th in terms of market cap against BTC.