As Tafia says above, ultimately, any property is only worth what a buyer will pay for it.
Sooner or later (as has been seen in western economies) house prices are affected by the ability of purchasers to finance them.
As a rather sweeping generalisation, property price rises tend to lead in good times and gain a momentum which carries them some way beyond what they are worth; however, when times go bad, property prices are resistant to falls as seller expectations and optimism strongly resist reality and thus will tend to remain high until sufficient sellers are forced into the position that they are forced to sell.
Sadly, for most current potential western buyers, I see little or no prospect for most of those who lost substantial amounts of money in their own country in the financial meltdown and who are also at the tail end of their earning years being able to recover their previous financial position.
Additionally for many of those same western buyers this has been exacerbated by the appreciation of the THB and/or depreciation of their own home currency – this can only get worse for those from countries that do not address the causes of the weakness of their currencies (e.g. an unsustainable fiscal deficit).
The combination of a substantial diminution in wealth from the financial meltdown in their own currency and the impact of a major shift in the exchange rate for many westerners will inevitably result in fewer western buyers over the coming years.
Despite protestations from some to the contrary, Thailand is not disconnected from what is happening economically in the rest of the world, and in my view prices will fall – but not for some time (unless you are lucky enough to find a seller that must sell).
However, having said that, there is the grey area to be considered as to whether the ability and desire of the domestic market might be sufficient to maintain property prices at their current level -this is an area that I really don’t have a clue
