A query is normally associated with various costs
1. Access Cost
2. Storage Cost
3. Computation Cost
4. Communication Cost
To estimate the cost it needs some information , this information could be got through catalogs. Catalogs stored all kinds of statistics which are needsed by this cost functions to find out which the cost associated with the query.
So in the paper I can still use the concept of cost of a query in a sensor network and the statistics are nothing but the parameters (characteristics) of the sensor network.`