Western men are rich
Most Thai girls believe Western men to be rich, and has already been stated, this is often the case in terms of relating the average earnings of a Westerner with the average Thai male. In some cases the woman will agree to the marriage simply for the money, and such marriages are certainly not made in heaven. They often end in tears and the loss of a great deal of cash by the man.
However, you should be able to tell if the girl you have chosen feels the same about you. Thai women fall in love with western men just as they do with young people from their own country, and while the initial attraction might have been your wallet, it might not be why they marry you. You should take the same time to get to know them as you would a girl back home. Many American and European women will fall for a very rich guy - you just have to learn to tell the difference!