you are closed to the right figure in USD.
People might not really be reacting to the news as pertaining the suggested immigration protocols, maybe the $9 might probably be an extra budget in the journey.
So economics management can also make them lament. Although might not be too much to be afford as tourist.
In the other side could be a means of the people protesting because the people are not comfortable with how governments just keep adjust taxes anyhow they want which could somehow give a frawning face to the Thailand's tourist destinations.
OP mentioned like people are reacting on this news, it is like more people are affected on it. I think Thailand got cost of living cheaper and maybe the usual cost to travel here is also cheap. So most people who travel here might also be poor. That is why. Still, so, tax is another issue in Thailand apart from this? But we now know on why this had happened. Because they already had a history. This is overkill for the poor.
Tax alone is already intended for the development of a country though and caters other things too. OP, this is a change, so it causes paper work and this change can also affect the tourism sector of Thailand. Definitely a bad idea. If other countries/governments are not selfish, they won't follow suit.