My old friend was on lower Sukhumvit many years ago where we both lived. At one of the sidewalk stalls a farang spent what seemed like 15 minutes trying to squeeze another 10 Baht reduction from the stallholder, who got so annoyed that he told the guy to **** off.
In my last 10 years living there I never bought things where haggling was involved, I just found it too tiresome in the end.
When I did haggle, if they said 200, I would say 100, and we would agree on 150. That was my target, suggest 50%, and settle at 75% of the quoted price. I'm not gonna spend 20 minutes in the heat and pollution and crowds trying to stiff someone for 10 Baht (around 20p). Thailand being cheap Charlie-central, many will do so.