At the risk of adding fuel to the fire for those that like to shoot down the Filipinos (or other NNES) teaching staff - be careful.
Yes, there are Filipinos (and others) who have no business here. Their language skills are substandard. Their TOEIC scores are in the 500-600 range. They are NOT teachers. They have NO teaching qualification of any kind.
They get hired simply because they can speak English better than the Thai staff at the school who hired them or they get hired by dodgy agencies over the next candidate who is qualified simply because they are cheaper but the agencies hire them and pass them off as "English teachers" anyway.
On the flip (no pun intended) side of that coin are those Filipinos who are qualified. They are trained teachers (degrees or post-grad diplomas in educational fields) with a good understanding of pedagogy, classroom management skills and and an excellent command of English (often better than the NES tourist teachers who abound in Thailand).
To those unqualified teachers... whatever your passport color, whether you are paid 30k for your "correct passport" or 15k for your lack of one, you are overpaid and have no business near a classroom.
To those qualified teachers with teaching qualifications, teaching licenses, 750-990 TOEIC scores, etc) - you should be paid better. 15,000 is too low (unless benefits like housing, significant vacation time (with pay) and bonuses are added onto that).