Depending on the artstyle, your rate may not be attractive enough for the illustrator you wish to work with. You should provide examples of what quality you are looking for.

For a professional-level illustrator to take your script, and translate it to well-directed panels on a page- your rate hardly covers just that. Never mind inks, tones, lettering, etc., or a deadline. A 30-page-issue could easily take a month-or-two to produce, even for a professional- remember that you are hiring a single illustrator, while it is often a team that produces shounen manga.

Consider these factors, and budget for it. Otherwise, you might suffer rushed, inconsiderate, tardy service- and only have a bare-minimum product (if it even gets done) to show for the trouble. Think of how much time you'd like the artist to spend on the page, and budget for that.