With the high selling prices of units in newly launched projects or brand-new buildings in Thailand and especially Bangkok, some buyers may opt to purchase more affordable units in older developments. On a per-square-metre basis, the prices of some of these units could be as low as half of those commanded by units in newly completed buildings or those under construction.
Findings from Jones Lang LaSalles recent market study indicate that newly completed or luxury units now being built in central Bangkok are being offered for sale at prices ranging between 160,000 and 350,000 baht per square metre, whereas units in older luxury buildings aged 10 years or more in the same area are available at between 90,000 and 120,000 baht per square metre.
Although there are still compelling reasons to buy for investment, buying a used luxury condominium in the centre of the city as a residence is now a sound decision. The strong competition in the leasing market due to the rapid growth of new condominium supply has put downward pressure on rents.

Many owners who bought condominiums for investment purposes many years ago may now find it difficult to let their units as most tenants prefer newer buildings. For this reason, certain investors are keen to divest their holdings. As a result, many luxury units are available at attractive prices and offer good value for money to those looking to buy condos to live in.
As mentioned earlier, there are still opportunities to purchase an older condo as an investment. Units in certain buildings, as old as 20 years, that are well designed, well located and well managed, continue to enjoy high demand from tenants. They also have the potential to enjoy capital appreciation over the long term as sites available for new developments are becoming scarce, while development costs continue to rise.
via Bangkok Post : Older condos can reap big rewards.