彼が持っているのは二百円です。 Supposedly this means "It is 200 yen that he has.". But I am really not familiar with this sentence construction. Therefore, I'm not sure how everything before の works with the rest of the sentence. Or what の is doing in the first place.
So literally, it would be, "As for what he is holding, it is 200 yen" - "As for what he has, it is 200 yen" - "As for what he has, it's 200 yen" - "He has 200 yen"
の turns 彼が持っている into a noun phrase, while は means "as for".
In natural English, it becomes "He has 200 yen." Without context, I cannot ascertain what is emphasized -see comments below as well as this question:
AはB emphasizing B, rather than A/aはb-emphasizing-b-rather-than-a
Depending on context, perhaps an implied meaning would be, "All he has is 200 yen."