We paid about 110k bht for a solar system fitted about 3/4 years ago. I grudged paying that when you can get a very efficient electric water heating system fitted for a fraction of that and electric is cheap when used carefully.
Don't know much about them but all I wanted was hot water on demand.
Frankly, unless your planted there the cost benefits dont really come in to it. Quite high maintenance cost as well after guarantee runs out and almost impossible to find someone if a panel is cracked. Our roof is constantly bombarded with bones//??!!
Of course you could be going down the environmental route.............
Shouldn't be in Thailand then!