3 dives for 3500-4000 baht. additionally, you have to pay the phi phi national park fee which is 600 baht per day for divers and 400 baht per day for non-divers. visit this link for more info https://www.sharkeyscuba.com/Trips_By_Day.htm
There's a beginner program called Discover Scuba diving. This usually costs 3500 baht (8k approx) in most dive centers. This includes 2 dives plus lunch in between the dives. If you want underwater photos and videos, it costs 1000 baht (could be shared per group).
Scuba diving is cheaper in Phi Phi islands than in Phuket and I've also read that the underwater clarity is better in Phi Phi.