If you think you have been overcharged, then ask the agent to provide a breakdown of the taxes. The amount you mention should pretty close to reality to me. The class of travel is not relevant for SYD-BKK return.

Some of the big ticket tax/levy items are:

Departure tax $38
Passenger servce charge @ SYD $41.16
SYD Noise Levy $3.40
Fuel Surcharge 2x$60 = $120
Thailand Departure Tax $1.80

So those add up to $204.80 and there could esily be a few more that I missed.