Hi, Aunt T. :)
Much of their value lies in their scarcity: unused planners are likely no longer being produced, and fans of Joni (whose popularity with those involved with the 'Jesus Movement' can hardly be overstated) are collecting them as memorabilia.
Paper goods such as planners, date books and callendars are not only fragile, but are usually thrown out after a years use, so relatively few survive when compared to, say, comic books.
Also, with the exception of the Bible itself, publishers catering to evangelical Christian customers typically print far fewer such items than do publishers of books geared to a wider clientelle.
(eg. Although they're not that dissimilar in subject matter, Stephen King's "The Stand" has far outsold all the books in the "Left Behind" series put together, and will continue to do so.)
Have I answered your question to your satisfaction? I hope so. :)
BTW, my friend, Nancy Honeytree, was so impressed by Joni's testimony (and Joni herself, of course) that she composed a song especially for Joni entitled "Joni's Waltz". Have you heard it, by any chance?
Yours,
~Seven