@ swissie:

Maximum fine for failure to do the 90 day report is 5000 baht. There is NO additional fine of 200 baht per day.

Taken from thaivisa.com/forum/topic/611007-if-90-day-report-late-get-red-stamp-or-anything/page-2

"the time window is one week before and one week after the day 90 days ends. After that the fine is of 2000 baht if you go to an immigration office to report it, and 5000 bahts if an officer check your passport and your 90 days report is over. There's not any additional daily fine"

From Immigration Act B.E. 2522 English translation,

Section 76 : Any alien, alien, who fails to comply with the provisions of Section 37(2),(3),(4)or(5) shall be punished with a fine not exceeding 5,000 Baht and with and additional fine not exceeding 200 Baht for each day which passes until the law is complied with.

From: Notification of staying over 90 days

4. The notification must be made within 15 days before or after 7 days the period of 90 days expires.

I made an honest mistake: I've always thought it was 200 baht per day, with a maximum of 5000 baht for failure to do the 90 day report (compared to 500 baht per day, with a maximum of 20,000 baht for overstay).

Therefore, I stand corrected. But I have never heard of anyone being forced to pay 5000 baht PLUS x days times 200 baht. Maybe it is never being enforced?

However, re: notification, I do know that it is 15 days before, not 7. I made a mistake to quote the wrong information.