"The plane ticket to Italy is too expensive!"
there are two versions of this sentence, one only accepts 飞机票 and the other only accepts 机票
One can drop "飛" in the answer because it's redundant; "去義大利的機票太貴了！" (using the Taiwanese variant of "Italy") is accepted.
去意大利的机票太贵 should be accepted. The 了 is only used for emphasis and is not necessary.
I think it is needed because of the use of 太, rather than for emphasis.
In Chinese, the attributive (along with its marker 的) is always put before the noun, which is different from English.
And here "to Italy" is the attributive.