It seems that while people are willing to bet on existing (query, url)
pairs, few are interested in adding new (query, url) pairs.

So maybe I could provide some kind of reward for adding new (query,
url) pairs.

One way to do this is to reward you for adding "interesting" (query,
url) pairs. For example, you might get a number of points equal to the
number of active predictions (both higher and lower) for your (query,
url) pair.  Note that you can't lose points; you can only gain points
here. However, the number of points you get could go down over time if
people lose interest in the (query, url) pair.

Of course, another way to do this is to allow you to "sell" your
(query, url) pair much like deleting a prediction. That way, your score
won't go down after you sell it.

Amir

P.S.  It may also be the case that a reward is not needed.  For
example, perhaps a bit of guidance is all that is required to motivate
people to add new (query, url) pairs. Ideas for new (query, url) pairs
might come from recent hot topics in blogs for example:
http://www.daypop.com/top/