Actually it is quite easy to get the retirement extension to a Non O visa, basically it is a proof of funds 800,000THB in a Thai a/c for at least 2 months (3 months for the next one) or 65,000THB a month or a combination of the two. Cost is less than 2,000THB. No need for an agent to rip you off for thousands.
I might be wrong but I think that rule changed a while back, now new applicants have to have individual amounts, of course you can use the 800,000 and stagger your applications ! Best check with the visa part of www.thaivisa .com