(number) bucks

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

I got this on sale for only four bucks.

get (something) on sale

When something is "on sale", it is being sold at a lower price than usual. In this expression, people use the verb "get" instead of "buy" to express buying something while it is "on sale":

I always buy my clothes on sale. I never pay full price.

"Getting something on sale" is usually considered a good thing, and some people like to brag about the low prices they paid for things.

(buy something) for (an amount)

When you want to tell the price of something that you bought, you say that you bought if "for ___":

I bought that car for two thousand dollars ten years ago, and it's still running.

But, if you use the word "cost", you shouldn't use "for":

This car cost me fifteen thousand dollars.