It is not a "bad" idea in itself... People have been trying to motivate others to do good (or even creative) work by paying them. It (almost) always failed. Why? Human nature? Perhaps. Great philosophers have rumbled about this same issue, the one that comes to my mind is Arthur Schopenhauer in his "Parerga und Paralipomena" discussing copyright and how pernicious it is (hey, that was back in XIX century, a real visionary...). Here is a quote from the English translation:
There are, first of all, two kinds of authors: those who write for the subject’s sake, and those who write for writing’s sake. The first kind have had thoughts or experiences which seem to them worth communicating, while the second kind need money and consequently write for money. They think in order to write, and they may be recognised by their spinning out their thoughts to the greatest possible length, and also by the way they work out their thoughts, which are half-true, perverse, forced, and vacillating; then also by their love of evasion, so that they may seem what they are not; and this is why their writing is lacking in definiteness and clearness.
Consequently, it is soon recognised that they write for the sake of filling up the paper, and this is the case sometimes with the best authors; for example, in parts of Lessing’s Dramaturgie, and even in many of Jean Paul’s romances. As soon as this is perceived the book should be thrown away, for time is precious. As a matter of fact, the author is cheating the reader as soon as he writes for the sake of filling up paper; because his pretext for writing is that he has something to impart. Writing for money and preservation of copyright are, at bottom, the ruin of literature. It is only the man who writes absolutely for the sake of the subject that writes anything worth writing. What an inestimable advantage it would be, if, in every branch of literature, there existed only a few but excellent books! This can never come to pass so long as money is to be made by writing. It seems as if money lay under a curse, for every author deteriorates directly he writes in any way for the sake of money. The best works of great men all come from the time when they had to write either for nothing or for very little pay. This is confirmed by the Spanish proverb: honra y provecho no caben en un saco (Honour and money are not to be found in the same purse). The deplorable condition of the literature of to-day, both in Germany and other countries, is due to the fact that books are written for the sake of earning money. Every one who is in want of money sits down and writes a book, and the public is stupid enough to buy it. The secondary effect of this is the ruin of language.
A great number of bad authors eke out their existence entirely by the foolishness of the public, which only will read what has just been printed. I refer to journalists, who have been appropriately so-called. In other words, it would be “day labourer.
... Parerga und Paralipomena, On Autorship and Style (circa 1852)
Now, of course, there are ways to reward contributors, and if the reputation system really work, I believe that there could be some kind of reward, but I still would try to avoid financial rewards, because even a great contributor, if paid by quantity of output, not quality, will be tempted to write as many answers as possible, certainly lowering overall quality...