The order of currency name and its numerical value (i.e. 人民币五十元 vs 五十元人民币)
In '人民币九十一元',  '人民币' modifies '九十一元'- (91 dollar --> 91 dollars in RMB)

In '九十一元人民币', '九十一元' modifies '人民币'- (RMB --> 91 dollars of RMB)

They mean the same, and you can use either one.

In '一美元价值8.2元人民币。', '一(元)' modifies 美元 (USD --> 1 (dollar) of USD); '8.2元' modifies '人民币' (RMB --> 8.2 dollars of RMB)