How can I formulate a universal theme that focuses on the idea that money has limitations?

Certainly, you could consider the old adage that Money cannot buy happiness. This is certainly true for Gatsby, and even Daisy and Tom.  They are incredibly rich, but Gatsby can never have what he really wants, no matter how much money he acquires.  He wants to rekindle exactly the same romance he shared with Daisy prior to her marriage and her becoming a mother.  This is impossible.  Tom and Daisy ultimately don't seem very happy either.  He has numerous affairs, making her miserable and implying that he isn't satisfied with her.  She has a tumultuous affair with Gatsby, indicating that her emotional needs aren't being met.  No matter how rich they are, they are still unhappy.

You might also consider the idea that Although one can acquire money, it is much more difficult to change one's status.  Gatsby doesn't seem to understand the difference between new money and old money.  His money simply isn't as good as the money the Buchanans have because he had to work for it (and illegal work to boot).  Tom has inherited his money, and this carries with it a different status, a status that Gatsby can never achieve and thus can never offer Daisy.  It may be possible for a person to earn a great deal of money, but it is another thing entirely to be able to change one's status, and this proves to be impossible in the text.