On test day, I was under so much pressure that my brain went on auto-pilot and my ability to methodically break down each question went out the window, even though I'd spent hundreds of hours studying similar questions at home. If your goal is to start law school in 2020, I think it'd be worth it for you to experience that pressure before the do-or-die January exam. I started with a low 140s diagnostic and ended up with a 163 after a year of studying.
$75k is a lot of money. However, what's "manageable" isn't really something we can answer for you. It depends on many things, like your salary as a student/jr. lawyer, your risk tolerance, the type of lifestyle you're willing to maintain while paying off debt, parental or spousal support, etc.