Prices vary only party because of supply/demand factors and assumptions about what the market can bear, but to a much greater extent due to the cost incurred by the boat operator to run trips (overhead). On Bali, a lot of the diving is near to shore, so the overhead for actually running boats is much, much lower than it is on Phuket where the dive sites are some distance away, meaning that fuel costs (one of the main cost items for our trips) are much, much higher. These costs are borne by the customer, ultimately, because the prices charged have to cover all costs.
For liveaboard diving starting from the Phuket region, there are a couple of two- and three- day options that offer good value for money. For daytrip diving, the most economical rates are for Phuket area local diving rather than daytrips to the Similans, which are a very long way away. If you are looking for the best deals, try to get trips with 3 dives a day since the price-per-dive ends up being significantly lower (the fuel overhead costs are pretty much the same for a given destination regardless of the number of dives offered). My personal favorite three-dive day is Shark Point/Anemone Reef/Koh Doc Mai. But it's not available every day. Another good three-tank trip goes to Phi Phi, and a third visits Racha Noi and Racha Yai. There are three-tank trips going to the King Cruiser wreck and two other sites as well.
Hope that helps!
