Yes, nyang = yang = ryang. When there is another syllable/number before this word, (like 일냥 (1 nyang), then it is nyang. But when this is the first syllable/sound in a word or when it’s by itself, it becomes yang because of 두음법칙 (The initial sound law). But actually Nyang is more commonly known and some people also use Nyang by itself, without changing it to Yang.
Although it’s hard to tell exactly how much nyang is but I don’t believe it was that much. I think of it as a small coin’s worth: i.e. less than $1. It’s possible that 1 won was 5 nyang when it was first converted. But at the same time, 1 won at that time was worth a lot more money than what it is now. In today’s standard,1 won is worthless - even less than a penny.
I read somewhere that towards the end of Joseon dynasty, you could buy a sack of rice with 8 nyang, which would have been worth about 150,000 won ($150 USD) in today’s money. So maybe saying that 1 nyang is about 20 USD is more accurate. I think I’ll go ahead and use this info on my document. 