Basically, financial proof is not required for dependent on a foreigner who is working in Thailand legally. Here is the requirements list for initial Non-O dependent visa, that is on the Immigration bureau HQ website. But details vary on each office. You surely need to get the latest handout from your local immigration office.
private company
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/4.FOREIGNER-WHO-IS-WORKING-IN-A-COMPANY-OR-LIMITED-PARTNERSHIP-NON-O.pdf
teacher
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/5.WORKING-AS-TEACHER-IN-THAILAND-NON-O.pdf