我一個晚上開銷就不只三萬了. What's 了 doing?
It really depends on the conversation context. The difference is too subtle to describe to people from different cultures.
It is hard to use your sentence directly to build an example. Suppose a client is bargaining/discussing some expense or price with you.
When there is no "了", you are probably just stating the fact.
When there is a "了", it is likely your tone is dropping during talking, you could be rolling your eyes too. You are indicating the client is too ignorant about the real expense you need and his estimation is way too low.
Or you could say it with a "了" in a tone of something close to "撒娇", indicating the client should pay a bit more than what he offered. In this case, you could probably or will intentionally choose to use "啦" instead of "了".
If it is a complete sentence: With a 了, it sounds more casual. The speaker may be rich. Without 了, it sounds rude, like you are arguing with someone.