I am quite surprised that no one has answered this question yet.
The two types of microfiche available are silver or vesicular,
the later being 15% less than the silver price, which is the
one quoted in the UMI "Serials in Microform" catalog, whose
1996 edition includes this information on page 15.

Silver film has greater longevity than vesicular, but I'll
leave that aspect to experts in preservation.

            Keith R. Stetson             |      (203) 254-4044 x 2184
   Collection Development Librarian      |      (203) 254-4135  (fax)
Nyselius Library, Fairfield University   |
       Fairfield, CT  06430-5195         |   kstetson@fair1.fairfield.edu