I assume you´re talking temporary import, not bring them in to stay?
At the BKK airport it was pretty straightforward (though I did have a thai-speaking person with me, which helped a lot!) and while I do not remember the exact amount, it wasnt very expensive at all.
I was adviced not to even bring the carnet with me
(Thailand is not in this system) and I didnt, but there were some copies of it within the freight papers, and I think thats where they looked up the value of the bike, which then corresponds to the amount of baht that will be put into the import paper. This is the amount you´re supposed to pay, unless you take the vehicle out of the country within 30 days.
In our case, the value in the carnet was 9000 euros, and the amount they put in the paper was nearly 23000 euros!! I did ask a few times, that I do NOT have to pay this, unless I fail to comply with the regulations... later I heard these taxes&duties can be avoided for about 6 months.
I also heard warnings not to use Bangkok seaport, whatever its name was, said the customs there can be a big problem. At the airport it wasnt.