@Rosie ********
I haven't prices them lately. My wife's AIS unlimited internet is $20 usd /month. My unlimited 5G is $30 a month. I think similar packages in the US will cost $$$$, so 'expensive' is relative.
I couldn't speak to Koh Phangan. I have never been there.
I was on one side of Koh Sumai last weekend. Every where I went the internet was pretty good. Occasionally I would find myself in a building on the side of the hill, which would get shotty internet, but mostly the internet was good.
Koh Tao had good internet on half of the island. The other half of the island was sparsely populated so there weren't many towers, no internet. If you stay were people live the internet was quite good. This was about 2 years ago.
I imagine Phangan is about the same.