Can´t remember exactly, but I don´t think we had to pay more than ~2000 baht, and that includes everything at the Thai end of the transportation. Took about 2,5 hours to get the bike out of the cargo-area.
We did, however have a thai-speaking person with us, plus we got great help from my friend, who lives near Bangkok, and has brought bikes into Thailand for decades, so I think that made it a lot easier for us. Seemed to me like it could be done without any external help, but could be a bit more time-consuming. They spoke very limited English.
They´re not in the carnet-system, so they should give you a temporary import paper of their own, we got 30 days (but Ive read something like this could be only 15 days now...) Anyway, if needed, you should be able to make a visa-run to Cambodia, Laos or Malaysia, and get a new temp import paper for the bike as well. I heard you could keep the bike in Thailand about 6 months until the customs really get nasty with you (but did not try this myself, so cannot confirm).
Compared to India, the Thai bureaucracy was a breeze, the only real problem was the alphabets, that look like worms to us... but most official papers were bilingual.