>Asking an American how much they earn or how much they paid for something can be perceived as rude.

American here. Really? I agree about the first part (how much you earn), but not the second. Lots of people like to brag about how they scored a great bargain on something, or complain about how something cost too much. But it really depends on a lot of things: what it is, what your relationship to the other person is, etc. You wouldn't just ask some stranger on the street how much their phone costs, for instance.

> Lots of people like to brag about how they scored a great bargain on something

But if I ask how much they paid for something, and it was not a bargain, will they feel bad? Like I'm mocking them for their poor haggling skills or something?

I don't have a lot of experience with Americans. It's just something I heard from my English teacher.

It really depends on a lot of things. And obviously there's a big difference between asking someone, and them volunteering the information to you.

I wouldn't ask someone how much they paid for a car, for instance: it's a big-ticket item, and strangely enough it's still haggled over in this country even though most things are fixed-price. Same goes for a house. But something small, I might. Retail goods are usually about the same price everywhere, unless there was a sale or someone took advantage of some kind of promotion, so it's not like the price they likely paid is a big secret. If I want to know the price of the latest iPhone, I can look it up. So you're not going to be exposing anyone's poor haggling skills by asking how much they paid for a (normally) fixed-price item. But yeah, I wouldn't ask how much they paid for a house or car.