The majority of periodicals that have been published over the years aren't worth very much, there are exceptions. Often a collection of books on a particular subject or area of interest contains periodicals relating to that subject and should be included in the appraisal regardless of their individual values.
In most cases periodicals are not accounted for individually as part of an appraisal but as a group or run, as in 10 or 20 (or longer) run of a professional journal. As a general rule, the more highly specialized a professional journal the more it is worth. Bound journals are of greater value then unbound ones. Professional journals are generally worth far more than most magazines aimed towards a popular market. Runs of professional journals should always be appraised for insurance or tax purposes.
In many cases magazines and other periodicals from the 19th century have significant value, especially if they are well illustrated (particularly in color) or contain important information for scholars, physicians, or scientists.
If you have a collection of periodicals, including professional journals, scientific bulletins, reports, abstracts, or related material and are interested in having them appraised please contact me: firstname.lastname@example.org