In reality if you were alone and willing to live in a place that was either farther from Sairee (where the dive shop, restaurant and bars were), didn’t have A/C, or wasn’t as new then you could easily find something in the 6,000-9,000 baht a month range. In fact for 9,000 a month you could get a nice house with a kitchen, bathroom, living room, bedroom, balcony and Wi-Fi that was a 5-10 minute ride from Sairee on a scooter. 6,000 baht a month would get you a little hut within walking distance of Sairee with a bedroom and bathroom along with a small front porch.
You could also reduce your overall expenses by partying less than I did and forgoing the western meals here and there. There’s also the option that if you will be in a location for a while and you have the money that you buy a used scooter at the start for 7,000 – 12,00 baht. This can save you in the long run because A. it’s your own scooter, therefore if you crash it you dont have to pay an outrageous amount to the owner to get it fixed, rather you can take it to a local mechanic who will be 1/10th of the price. B. when youre finished with the scooter you can resell it for roughly the purchase price, thereby recouping your initial cost.
For an item by item cost break down I recommend using Numbeo Thailand, it allows you to pick specific cities and islands so that you can get an idea of what your monthly cost of living in Thailand will be.