I've lived in SE Asia for a few years and, yes, you are right - this monopoly route is one of the most expensive I've ever seen. I also agree that flying in/out of Surat Thani is a pain in the ... Bangkok Airways also owns the airports in Sukhothai and Trat (Ko Chang), so you might find sky-high prices there, too, but there are better alternatives.
And I do agree with some other posters that it's easy to get used to/spoiled by the low fares around SE Asia. I've done so many 3h+ international flights for under $100 that I now gasp at $300rt fares...
If somebody doesn't like the high fares to Ko Samui, your best bet is to book early, book through one of their countless partners (which might result in a cheaper fare for the same seat) or go somewhere else. Ko Samui is nice, but there are much nicer beaches in Thailand...