Flagship phones for other brands are usually in the $700 range. So it's cheap for a flagship at least..
'Cheap' is used in a specific context here. The headline says 'how cheap a _flagship_ phone can be'.
There are cheaper phones out there. Would you name them among 'flagship' devices?
What's the minimum spec to be considered "flagship?"
Well it's not a lot (as in 'a lot lot') of money for large amounts of people, especially amongst those who would buy a phone that you can only buy online and for which you need to go through an 'invite' system. "Cheap" is always a relative concept, so I'm not sure what it is you're complaining about.
It's cheap for what you get. Try comparing it to an iPhone, specs and price.