$300 plus shipping and taxes. In your region. And a whole lot more than $0, which is the cost of staying on someone's old phone. when someone's buying a new phone already, considering its compatibility with Lineage or Graphene is something that should be on more people's radar, I agree. But switching from googled vendor'd Android to fully open Android isn't a pure skill issue like switching from Chrome to Firefox (/Waterfox/librewolf) or Windows to Linux is. "I'd switch but it's too hard" is a much smaller reason than "I'd switch but it's too expensive" is.
Someone's five year old phone is just as likely to be a five year old Samsung/etc with a locked bootloader.