for the sake of discussion, (sentence)

When you don't agree with someone in a discussion or debate, this expression can be useful. You use it to continue the discussion, even though you don't agree about one point.

For example, you can use it if you and a friend disagree about whether to fly or take a train to travel to another city. You think the train is faster, but your friend thinks it's faster to fly. You can't agree about this, so you don't want to argue about it. But you have other points to make too, so you say:

For the sake of discussion, let's say that flying is faster. The train is still about half the cost.

In this example, you don't really think that flying is faster. You think it's slower, but you want to move on to another reason why the train is better.

let's say that (clause)

When you're discussing a topic, sometimes you have to imagine a situation and then describe what would happen in that situation. You can do this with the phrase "Let's say that..." Here's an example:

Let's say that I hire you at my company and you do a bad job. Should I have to keep paying you, or should I be allowed to fire you and hire someone better?

In this example, the speaker isn't really hiring the other person. In fact, the speaker might not even have a company! The speaker and listener are just imagining what would happen in that situation.

Here are some more examples:

Let's say that someone breaks into your house and tries to attack you. What would you do?

Let's say you ask her out and she says "no". So what? It's not the end of the world.

This expression is really useful for debating or explaining things. 

(number) bucks

"Bucks" is a slang term for "dollars". It's used in casual conversation.

I got this on sale for only four bucks.