[RadioKid]

As you know, western numbers are expressed in thousands while numbers are expressed "10000" or "MAN:万" ​in Japanese.

For Big numbers like as 123,000,000 (One-twenty three million) could be expressed "123百万" but "十八千" will not be understood in verbal communication. Even in written form, "十八千" is confusing. Rather, "18K" is better but 18K will cause another confusion with the grade of gold.