"a buck" basically means "one dollar". But when I saw a movie, someone says "a buck fifty" while the subtitle says "150000 dollars". (It's understandable because it's a cost to fix a zoo.) That means, "a buck" can sometimes means "100000 dollars", right? But why?

So you’re right. Normally a “buck” is $1, and a “buck fifty” is $1.50.

However, we sometimes abbreviate large numbers. This is most common in casual, spoken speech.

For example, if I say:
“I’m looking to buy a house in the 250-300 range.” I actually mean that my budget is $250,000-$300,000. Why? Because in the context of houses, this makes the most sense.

Another example: When I play cards, most players are gambling a few hundred dollars. So sometimes we call $25 a “quarter” (when really a quarter means $0.25, 25 cents). Here, a “buck fifty” might mean $150. This is just because $1 makes no sense here.

So in short, a “buck” can mean $100,000. But it can also mean $1,000 or $1,000,000. You have to think about the context and decide.