The price discussion is interesting. We don't have a speedboat, but I have worked for other companies who do organize speedboat trips to the Similans.
Guests want to get there fast so that they have time to do two dives and have lunch, while still returning to their hotel in time for cocktail hour. Guests also don't want to be cramped together with 30 other divers. Similans are approx 55 km west of Khao Lak. Speedboats have 2-4 min 200 HP engines, generally 4-stroke. One liter of petrol/gasoline is approx. 40 Baht (I know more about diesel prices these days). The boats take a lot of maintenance, since it's pretty tough on a boat to go more than 100 km per day at full speed. The Similans are only open 1st November through 30th April, so the boats only operates 6 months every year. National marine park fees are 600 per diver per day.
Do the math and you'll find that running a boat here is NOT a license to make money - quite the contrary: the price is well justified. Many companies struggle financially, especially now that less Europeans visit us.
If you want to visit Thailand, the flight is generally more expensive than going to your neighbor country. The same goes for diving: if you want to dive more remote locations, you'll pay more than if you do shore diving. Simple.
Happy diving
Karin