From a purely financial perspective, it sounds like your "break even" point is just after year 9, assuming flat salaries in both scenarios. That's a long time for a positive ROI, IMO.
Something else to consider: teachers have a high burn out rate. Many go into the profession only to leave within a year or three. My recommendation is to teach for a while with the certificate and then, if you are sure this is something you will be doing 10 years later, consider the benefits of the Masters.
There is this if I teach in a shortage area:
"34 CFR 674.53(c) enables Federal Perkins Loan borrowers who are full time teachers of mathematics,
science, foreign languages, bilingual education or any other field of expertise where the State educational agency determined there is a shortage of qualified teachers to qualify for cancellation of up to 100 percent of their loan;"
Emphasis mine, obviously. So, yes, if I can get a Perkins Loan, definitely. Fine print:
"(2) The borrower must be teaching full-time in a public or other nonprofit elementary or secondary school that—
(i) Is in a school district that qualified for funds, in that year, under part A of title I of the Elementary and Secondary Education Act of 1965, as amended; and
(ii) Has been selected by the Secretary based on a determination that more than 30 percent of the school's or educational service agency's total enrollment is made up of title I children.
(d) Cancellation rates. (1) To qualify for cancellation under paragraph (a), (b), or (c) of this section, a borrower must teach full-time for a complete academic year or its equivalent.
(2) Cancellation rates are—
(i) 15 percent of the original principal loan amount plus the interest on the unpaid balance accruing during the year of qualifying service, for each of the first and second years of full-time teaching;
(ii) 20 percent of the original principal loan amount, plus the interest on the unpaid balance accruing during the year of qualifying service, for each of the third and fourth years of full-time teaching; and
(iii) 30 percent of the original principal loan amount, plus the interest on the unpaid balance accruing during the year of qualifying service, for the fifth year of full-time teaching. "