Thailand and the Phillipines are great for places for continuing ed. Also the other living costs are relatively low. Phillipines may be a bit more affordable than Thailand (assuming you're going to Phuket), but that mainly depends on the type of traveller your are too
Student lodging is best since you'll be able to meet other divers. But you can still do the program while waiting for space in the dormitories. There are a few guest houses I stay at in Phuket when i go there by myself that range from 300-600Baht/day. They are clean & safe but you share a bathroom and you may not get air conditioning. But when I'm there w/ my mom, we stay at the Marriot or the Holiday Inn resort
Not sure about the SE Asia part during the months of June & July though. It's typhoon season.