Koh Tao is just a few hours away from Koh Samui by ferry. It has over 40 dive centres and you will pay around 9800B for an open water course. Rooms start from 400B per night. November is monsoon time and some dive centres do close over this period.
I live and work here as an underwater photographer so do feel free to PM me if you want more info...
I have cut and paste a response I wrote to another post below .... this might help you when looking for and selecting a dive centre - no matter where you go.....
I am an instructor and underwater photographer and I live on Koh Tao.
Firstly I always strongly recomend that would be divers do not choose their course based on price. You dont choose to bungy jump with the cheapest operator just because they were the cheapest.
Its far more important for you to choose the dive centre that is right for you so go and check them out and see where you like.
Think about the facilites and their equipment and boat - ask to see them and be shown around.
Check what is and whats not included....
Talk to the staff - see if you can talk to who will be your instructor - are they passionate / experienced and enthusiastic?
Find out what happens if you cant complete your course - do you get a refund?
If you are having problems with something - can you take an extra day and repeat or will you have to pay more?
How many people will be in your group? One instructor is allowed to take up to 8 people. Be aware that if there are 8 (or more if there are 2 instructors) in your group then the course does not take any longer so you will get less attention and personal development from the instructor than if you were in a smaller group. On three of your four dives you have excercises to complete and it takes a lot longer for 8 divers to do this - so much so that you might get very little time actually diving.
Think about accommodation too - make sure you are somewhere quiet - you'll want a good nights rest.
The Open Water Course last 3 and a half days and is around 9800B.
The Advanced Open Water Course takes another 2 days and is 8500B.
If you have anything specific you want to ask then you can PM me.