This is a good question. To answer the second part first: registering an ISSN should cost nothing, you do it through a deposit library or similar (we did it through the BL, for example). If you publish a serial and can demonstrate that it is not of merely local or ephemeral interest, they should assign you an ISSN. (Some registrars apparently will object to assigning an ISSN to a blog; see http://fawny.org/issn/compatibility/ for some discussion of and responses to such an objection.)
You first question, however, is less obvious. What do you gain by registering an ISSN? What does any serial gain from same? Recognition, a certain official status, the right to be archived and indexed by a deposit library, etc. To quote Joe Clark again, “Your blog will then officially exist in the worldwide standardized encyclopedia of periodicals.”
There may be more to discuss on this question. Tom, do you have anything to add?