By advertisement she offered him privileges whereby he could add eighteen lessons to his store for five hundred dollars more.

That is to say, he could get a total of thirty lessons in her college for eight hundred dollars.
Four thousand times eight hundred is--but it is a difficult sum for a cripple who has not been "demonstrated over" to cipher; let it go.
She taught "over" four thousand students in seven years.

"Over" is not definite, but it probably represents a non-paying surplus of learners over and above the paying four thousand.

Charity students, doubtless.

I think that as interesting an advertisement as has been printed since the romantic old days of the other buccaneers is this one from the Christian Science Journal for September, 1886: "MASSACHUSETTS METAPHYSICAL COLLEGE "Rev.MARY BAKER G.EDDY, PRESIDENT "571 Columbus Avenue, Boston "The collegiate course in Christian Science metaphysical healing includes twelve lessons.