Is THB 18,000 a high cost for an Extension of Stay for a dependant in Thailand, and what documents are needed?

Hello to you all, we received back in Europe Non Immigrant B visas. My husband is in process of getting his 1st extension of stay based on employment and Work Permit, all is paid by his company. We have just consulted the woman (outside accountant) about extension of stay for me - and she said that the cost would be THB 18,000. I find it rather expensive. We have obviously passports, photos, our marriage certificate translated into English. I am a dependant only, no employment. Is the price she gave us high? What documents can we prepare for Extension of Stay for a family member? Our Immigration Office is in Krabi. Can you please help, can we do it by ourselves (after my husband get Extension of Stay) and what docs are needed. Thank you