No publishing firm requires an MA for entry - what they massively value is work experience, which you should get as much of as possible. If you look at publishing vacancies (google for 'bookseller'), almost all of them require you to have spent about a year or so in publishing. I think you would be better off spending a year getting experience in publishing, be that work experience or paid (if you're very lucky) rather than spending all that money on an MA. How are you planning to fund it?