In the northern parts we say 六块四 for 6.4 dollars, in popular slang at least. The duolingo translation feels extremely weird
Most time, In the northern part of China, they use 块 instead of 元, and 毛 instead of 角
Not where I've lived, people generally round to the nearest yuan and sometimes you get people rounding to the nearest 5毛 for smaller sums of money (like paying for using a bike pump or something), and I haven't seen a 角 for years, so sticking with 元 should be fine.
As for the grammatical structure, if they did use the smaller currencies this is the way it would be said.