It's totally cultural. In the US, people who have enough money to get by don't talk about making or spending money. And they don't really want to either, because it allows people who make 60K a year and people who make 250K a year to both call themselves "middle class". Even close friends don't necessarily know each other's salaries.
But all is not lost. It's hard to find a startup job where you won't be working closely with people from cultures where discussing your salary, rent etc. is perfectly normal.
That's how I was able to discover that as a woman I was making 75% of what my male colleagues were making. That's a Pandora's box type discovery, and I did stew on it for a while.
But I liked the company, and I didn't want to leave, so I saved that information for the end-of-year talks. They ended up bringing my salary level with the others', and then I was asked politely not to do that again.
It was quite uncomfortable for my boss - despite what you may think, your boss probably doesn't remember your exact salary - and I think my salary was an oversight (I was one of the earlier employees) rather than an intentional slight or because I'm a woman.
But still - what are the chances that could have happened if I hadn't asked my colleagues?
Don't ask, don't get.