This is from the London Thai Embassy website FAQs :
"14. Can I go to Thailand if the validity of my passport is less than 6 months?
As a general rule, passports should have at least six months of validity when travelling internationally. Most countries will not permit a travel to enter their country, and will reject visa application, unless the passport will be expired at least six months after the final day of travel. So, if your passport has less than six months remaining until the expiration date, you should renew it right away.
However, if you are entitled to Thailand's tourist visa exemption scheme, may not notice the expiry of your passport until last minute, and decide to go to Thailand anyway, this is at your own risk. It is entirely the discretion of the airlines and the immigration officer whether or not to allow you on board the aircraft and enter Thailand, respectively.
Please make sure of the validity of your passport whenever you plan to travel. "
Believe it or not, Thai Embassy websites (run by the Ministry of Frieign Affairs) contain errors on Immigration matters (run by the Ministry of the Interior).
the requirements Wolf posted are the requirements for the first 12 month extension, you cannot get a new extension if you fail to meet these requirements plus the requirements for applying for your second extension (800k for 2 months or 65k/month for 12 months)