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We were using BUPA both for foreign and Thai staff, but found that they are very slow to pay out claims, and unnecessarily bureaucratic (the EXACT opposite of my experience with BUPA in Hong Kong, who were great!). We've switched to AIA since March, and found them to be much better (quick payment of three claims filed, and helpful staff).
As others have mentioned, there is a sliding scale based on age -- I'm in my late 30's, and I believe I paid just under 20,000 baht, for a plan which gives me coverage in Thailand as well as those countries I travel to frequently. For the latter I had to 'pull a few teeth' to get them to provide me with a list of AIA approved hospitals, but eventually they provided that information for me.