Why did Berk have a good laugh at Allan's expense?

（误译）为什么伯克大笑艾伦的花费？
（正译）为什么伯克对艾伦大加嘲笑？

1. laugh at 嘲笑；因……而发笑

I prefer you laugh at me to my face!
你要笑就当着我的面笑！

It is important to laugh at oneself.
对着你自己笑是重要的。

Oh, cheer up! Nobody will laugh at you.
哦，振作起来吧，没人会笑你的。

2. expense n. 损失，代价；消费；开支  vt. 向……收取费用  vi. 被花掉

at the expense of  以……为代价；由……支付费用

In that case the politicians' push for more trade may come at the expense of fair trade.
在这样的局面下，政客们推动的贸易量的增长只能以公平贸易作为代价。

It must also be fair, not allowing one task to run efficiently at the expense of another.
它也必须公平，不能因为要有效率地执行某项任务，而牺牲了另一项任务。

People sell it, at the expense of their leisure time, purely as a means to the end of consumption.
人们以自己的闲暇时间为代价出售它，仅仅是为了消费的手段。

at no expenseat  不花钱地；没有损失地

I will stop at no expense to obtain it.
我将不惜一切花费来得到它。

There is no happiness for people at the expense of other people.
为了自己而牺牲别人，不会不快乐。

If you cannot afford an attorney, one will be appointed for you at no expense.
如果无法负担律师费用，法院将为你指派一名辩护律师。