Hello Jonny,
please look up my article on this forum page which I posted on 18th August 2011 titled ' A word of warning to riders and buyers of big imported bikes ' which relates to my very recent experience of buying an imported bike with no green book. The article goes a long way in answering your question, and even further with the possible hurdles you will face. You will be able to get a green book but the cost will depend on the dealer getting it for you, and the engine size of the bike. For my bike, a 25 year old Honda Clubman 400cc., I will have to pay about 20,000 baht for a ' grey ' green book. I was quoted a minimum of 70,000 baht by a bike shop in Buriram. You will find it just about impossible to get the green book by yourself ! As lordofthedreadz wrote, you must have a Resident Permit to buy ( and sell ) any motorised vehicle in Thailand. These are available from your local Immigration Office -- current cost at Korat I.O., 500 baht and they are ONLY valid for 30 days. As the name of the document implies, you will need to supply an address. The full list of document copies you will need for a resident permit is as follows; address/blue book, passport, visa page, arrival card, passport size photos -- you will need 2 of everything. This is what I had to submit at Korat I.O about 4 weeks ago when I applied for a resident permit to sell a motorbike, things may vary from office to office !