There are a host of problem actually
1. If users can get paid to answer questions, why would they attempt to answer the ones that are not paid ?
2. Paying would increase 'homework' questions
3. Dispute over pay is a big headache
4. Who handles the manages the transaction ? Paypal ? Credit
cards ? Javaranch is a public forum. Private data is best left - private.
What motivates people in this forum is the willingness to help others. Petty money is unlikely to motivate this gregarious bunch.