Tanchat
Shouldn't it be "seven dollars that would have been spent on extinguishing big fires are saved" ? Is it acceptable to rearrange the structure?
Yes.
Many different kinds of modifiers
follow a noun. Verbs and verb phrases also
follow a noun (specifically, the noun that's the subject of the verb).
When a noun is followed by two or more such groups of words, the writer has to choose the order in which to write them. That decision is guided by an assortment of preferences and style rules—for which you're definitely not responsible on the GMAT (which does not test stylistics in any capacity)—and sometimes is purely arbitrary.
When a noun is followed by two word groups of starkly differing lengths (= one much longer than the other), a common convention is to choose the order that puts the
longer word group
LAST.
Both of the following sentences are ok:
You are everything to me.You are to me what rainfall is to thirsty plants in the desert. (This sentence follows the convention described above.)
If you don't understand the appeal of doing this, just think about what the sentence would look like with these phrases in the opposite order: "You are what rainfall is to thirsty plants in the desert to me." That's basically unreadable.
In the problem at hand, "are saved" works like "to me" above, in terms of its placement after the noun ("seven dollars").
What's important to realize here is that
you do NOT need to DECIDE WHEN to invert the 'usual' order[ of two groups of words that follow the same noun—because you aren't asked to WRITE these sentences.
You just need to realize that the order CAN be inverted, so that you can properly recognize either of the two resulting formats as potentially correct.