Well, honestly, that can change by the year, month and even day! Airlines start up and fail, open new cities, have sales and so on. So, I would check out the websites of the major carriers plus the newer, low-cost ones. You can even sign up for a daily/weekly/monthly e-mail bulletin with their sales and special fares. I get about a dozen various travel-related e-mails per day, including one I saw on Prague for $399 including 4 nights hotel that I will forward to the Boards, today.
Depending on your needs, a couple carriers do something called the All Asia Pass, or similar. For one price, you get unlimited travel to where they fly. I know that in the past, Garuda (Indonesian) Airlines, Cathay Pacific and I think Malaysian Air has offered this, so definitely check their sites.
All that said, Bangkok is a good city to find budget airfares.
Good luck, and feel free to ask more specifics...
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