What is the process and cost for obtaining a Non-O visa extension in Thailand?

A user shared their positive experience with their second entry under the DTV category, highlighting an enjoyable interaction with a female immigration officer. The discussion included various comments on the requirements and costs associated with extending a Non-O visa, including financial requirements like holding 400,000 THB in a Thai bank account for at least two months prior to application. Several commenters discussed experiences related to changing visa types, agent assistance, and differing costs for applications made in Thailand versus abroad.