There are many many dive centers on the island, but most have the same prices. The larger shops are Ban's, Big Blue, Sairee Cottage, and Crystal Dive.
After years and years the cost to dive recently went up but is still less expensive than you'll find in the US. Also, all of your equipment is provided at Ban's rather than having to buy some of it in the US.
Standard pricing across the island:
Discover Scuba Diving 2,500THB
Scuba Diver 8,500THB
Open Water Diver 11,000THB
Adventure Diver* 8,500THB
Advanced Open Water Diver * 10,000THB
Emergency First Response 4,000THB
Rescue Diver** 10,000THB
If you are a returning student, you will usually get a discount on your continuing education courses. (Ban's is 10% discount for previous students)
If you are already certified and just want to fun dive:
One dive 1,000THB
2+ dives range 700THB-900THB each
Most dive centers only have double tank dives meaning that you have to do a minimum of two dives. The cost will go down the more dives you do. Each dive center handles this differently. All that said, you can expect your fun dives to be between $20-$30USD each (remembering you will do two at a time).
*prerequisite Open Water Diver
**prerequisite Advanced Open Water Diver