What do I mean "for free?" I mean "without increasing data usage".
Currently the money on each character is an unsigned long integer, which is 4 bytes, a max total of 4,294,967,296 (in copper pieces), or ~4.29 million platinum pieces. Which is a lot, but not enough to trade with high end stuff. And I think high end stuff should be trade-able without needing to make multiple transactions with possibly multiple mule characters, it is absurd.
So this is what I propose to solve the problem AND make the game feel more fantasyish:
Divide types of money into pieces of one hundred.
That is to say, 1 plat = 100 gold = 10k silver = 1m copper, rather than 1 plat = 10 gold = 100 silver = 1k copper.
The way to represent this digitally is simple, one byte each for platinum, gold, silver and copper with a 1:100 conversion rate at each step.
The total you could have is thus 255 plat, which is worth 255m copper, a full 16 times higher amount, and with the added value of going back to using gold as the main currency and platinum would only be used in rare and special trades which would make the highest currency also FEEL rare and special (as it rightly SHOULD).