I'm trying to determine for a pretty simple e-learning course, some graphics, everything tailored to meet a government specification, 40 words per screen, the cost, per screen for design.

My past experience has been about $125 per screen. Does that sound right?

Sure...
Based on experience, I estimate what each 'type' of screen will take to create (simple = text/image, medium = text/image/animation/video, complex = interactive components) then do a count of how many of each screen.
Then multiply that by the developer's hourly rate.
Then pad it a little for 'just in cases'.
HTH
CM

How many screens? You should discount for volume.