这里的 one of 并不表示“之一”，而是表示“关于……的一个（问题）”，即这里的 one of money 其实相当于 a problem of money（关于钱的问题）。你的句子意思是：

The problem is not reducible to one of money. 这个问题不能简单地看作是关于钱的问题。

类似的例句又如：

(1) His life was one of pain and suffering. 他的一生充满了苦难。

(2) Lucy’s mood was one of deep depression. 露西的情绪极度消沉。

(3) His expression changed from surprise to one of amusement. 他的神情由惊变喜。

(4) The response to the jury’s verdict was one of outrage. 对陪审团裁决的反响是愤慨。

(5) The question which needs to be addressed is one of funding. 需要面对的是资金问题。

(6) The question became one of whether Man could control the disease he had invented. 现在的问题是，人类能否控制住这种人为的疾病。