你要表达的意思很清楚：在周末乘机度假通常是一周中最昂贵的.
这种句子涉及到形容词的最高级的问题.正确形式：
Trying to fly on weekend is always the most expensive in a week.
如果要用“more than”,正确形式：
Fly on weekend is always more expensive than on weekdays.