It's always interesting to hear what others on your tour have paid as the prices vary widely. Often those who had paid the most had booked through their hotel - which always a bad idea. As you say, there are hundreds of counters so if you don't get a good price then walk to the next one.

Out of season, I've had whole day trips for BHT500 or less which is a bargain considering the general costs of everything else on Phuket.