system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how is it that politicians (take your pick) make millions of dollars as a politician yet their salaries are largely under 400k annually? Am I missing something?

I came across a link where Chelsea Clinton (daughter of former president Bill Clinton) is buying a 10+ million dollar apartment in Manhattan. How does she have this kind of money? 

Mitt Romney makes sense as he was a successful business man prior to being in politics, but it seems like they reap a lot of other financial benefits no one bats an eye to. 

..and no one is concerned that Hilary is spending 2.5 billion on her campaign? (Not her money but still...excessive?)  

EDIT: people keep mentioning book deals, but they aren't writing Harry Potter novels here. I understand the books make money, but I wouldn't imagine they make them 10s of millions of dollars.
gpt
Most politicians have sources of income other than their salary. For example, speaking fees and book deals result in *massive* payouts. Every time a former President goes somewhere to speak, you can be damn sure they're getting tens of thousands of dollars to do so. 

Once you get to politics at the national level, there's pretty much no one who isn't already individually wealthy. Everyone has either run a businesses or worked as analysts or lawyers in high paying positions.