What are the costs of pursuing this dive?

: Minus $50,000 to $300,000+

First, why is there a negative number?  Simple: some people actually get paid to pursue doctorates!  Yes, it’s rare, but opportunities like that do exist out there in the form of fellowships and assistantships.  We’ll share some resources on that later.  
In the middle, you can attend tuition-free through scholarships.  
At the upper end, it could cost hundreds of thousands of dollars due to the terribly inflated and undeserved high tuition costs at so many universities these days.  But alas, it’s the world we live in.
Bottom line, achieving your terminal degree has a wide range of price points, and some that actually work out tremendously in your favor. 
The key?  Research.  Do your research up front to try to find the best path forward for your particular situation.