The Scientific American has a very interesting article on Money and Mind.
I believe money is definitely a motivator to some extent, but beyond a point, it loses its value as a motivating factor. Successful people have rarely used money as the prime motivating factor. I think they believed in the cause, or genuinely enjoyed the effort. Money was merely the by product.
I also believe that there has to be a right balance between rewarding effort and rewarding outcomes.
Hence I feel deeply distressed when the Singapore Government defends its high ministerial salaries and money becomes the sole determinant to judge people.