I started with this customs website.
Thai Customs
I was in Melbourne Australia at the time. My wife (Thai) made a couple of phones calls to customs. One guy did try to tell her that there was a 300% duty. Another call got her to a woman who basically said bullshit. She emailed me the calculation formula for the various taxes. The top duty rate is 80% of CIF (cost + insurance + freight). The website mentions discounts to the value (cost) based on how long you have owned the vehicle.
You first have to get the import approval from the Department of Commerce in BKK. You must go in personally to do this. Bring someone savvy in Thai with you. I had to jump through a few hoops because I was not prepared. But here is what I ended up having to provide:
* Passport with minimum 12 month visa.
* Vehicle registration history. Must show minimum 18 months ownership in your name. Alternately, 2 or more years registration certificates will suffice. I got a report from the roads department. I had to print and sign it and send by mail. Then had to forward the email I got from the roads dept., just to prove I did not make up the report myself.
* License history, also from the roads department. I had just renewed my license and they would not accept that I had had one for 18 months before the new one. Again, if you have saved old licenses those would suffice. Again, print, sign, mail and forward email.
* Invoice to show value, but when I gave it to them, they really didn't accept it. I had to go to the internet and download a price guide from NADA. It was a lower value than I gave them, so they accepted my value. haha. Now this value is for the import paperwork only, the final value for duties is decided in customs.
* Photo of the bike. Simple.
Long story short, have all this information together before you go. Plan for a day in BKK to fill out the paperwork. Bring a savvy Thai with you cause the forms are in Thai and they are not supposed to help you. Stay calm outside, it not inside as well. After you submit the application, you will get an email or phone call from the person who you dealt with. Maybe all is in order, maybe they request more documentation. My impression is, they are up on any scams or dodgy documents and it will only complicate things. Once all is approved by the manager of the office, you will be asked to return to BKK to fill out more paperwork to get the approval which is in the form of a plastic card with your photo and file number. This allows you to pickup your vehicle from customs. I think. This took another day.
Bike should ship from Australia this week along with household goods. You can only bring one bike, (I have 2) so I picked the one I love the most.
I expect to do the customs in 4 weeks or so. In preparation, I have downloaded ads showing prices for the same year bike from Facebook for sale pages. I have also downloaded a report from Hagerty.com, showing prices for bikes in similar years. This is so as to not leave it up to customs to set the value. If you have some documentation showing applicable prices, they should at least be close to that. This I got from a manager at customs.
I will update on the final outcome later.