My good friend I do not agree completely with the above. Yes, only 400,000 for an extension based on marriage is correct. However, from what I have heard and what I have seen its the 'easy peasy' bit that is not so simple.
I have seen guys trying to get an extension based on marriage go armed with their whole life story bound in a fat book 2 inches thick. The Immigration Officials seem to need everything but the kitchen sink to confirm that you are married, where you live, how long you have lived there, how much money you have, etc, etc. I have also seen many turned away and told to come back later when they have collected some more pieces of paper and photographs to confirm they are who they say they are. Even then, assuming that the package meets their needs, they are unable to issue an extension to the visa because it all has to be forwarded to Bangkok for approval. I understand they give a temporary extension of stay (30 days) and one has to return to Immigration for the visa. It is also known that sometimes one can return several times, 30 day extension each time, until the approval comes through from Bangkok.
In contrast, the visa extension based on retirement is very straight forward. Complete the correct forms, original letter from Bank manager or Embassy to confirm monies in the bank (800k) or income (65k per month), one recent passport photo of yourself, hand over 1,900 Baht to the official processing your application and you can be on your way within 20 minutes with a new 12-month extension in your passport. I find it is useful to take my wife along because sometimes they have asked for verification of where I live. She can produce the blue house book (tabian baan), sign an appropriate form to confirm my residence and everyone is happy.