If you schedule your appointment on an MDA clinic day, you may have little or no charge, as that office is an MDA clinic office.

I know back in 2001, when I went the initial office visit was $500. I have Medicare and Medicaid (which will not work outside of Vermont).
Before I went to my appointment, I called my local MDA to find out if I could get some help on the 20% Medicare would not cover.

Long story short, my $500 appointment, along with Medicare and MDA only cost me $45 out of pocket (not including travel expenses).

I also suggest calling your insurance company seeing how this would be "out of network". You have 2 very rare medical conditions.
With some paperwork put in to the insurance company prior to the Johns Hopkins visit by your local neuro, you may get "prior auth" and your visit may be covered.

Start making some phone calls, It never hurts. Frustrates, yes. Hurts, no.