I did a similar project once.  The equation goes something like
    this:
    (I did not post this to the rest of the list--if yo think
    others would find it useful, go ahead and do so.)

    First figure to determine: Cost of storage space.
    Take the total square feet of your entire facility (including
    exhibition and office space) and divid it by the total amount of
    rent/payments paid on the whole facility, including property
    taxes--this gives you a cost per square foot for all spaces at
    your museum.  Second, you determine the number of square feet in
    the storage facility portion and then useing your above cost per
    square foot figures, determine the cost per square foot
    for the storage space alone.  By the way, if you have large
    portions of your collection on permanent display, you will need to
    play with the figures to reflect an accurate space allotment.
    This gives you a cost per square foot figure for storage space
    only.

    Second Figure to determine: cost of professional staff, misc.
    Then you factor in other things:  Percentage of wages per hour to
    care for the collection (including paper work, etc.); cost of
    appropriate shelving and racks to store the work; insurance
    costs; and so forth.  You will need to use your best judgement in
    some cases (remember, this is only an estimate). Add this amount
    to your first amount.

    Third figure to determine: Cost of care for each object, on
    average.
    Finally, you divide this above figure by the number of
    items in your collection and thus you determine an average cost
    for care per object per year.  It is actually kind of a fun
    project to undertake, but also time consuming.

    John Handley