How do we calculate the cost of a Query? could it be calculated in numbers? then what is the unit of measurement?
The unit primarily depends on the purpose for what you need this calculation.
For example, for one project we were asked to calculate on the basis of type of joins. For another, it was based on time. So it depends on requirements.
If you are considering costs related to resource utilization whether it is processing time or CPU, the Database Query Log (DBQL) can be configured to collect all that detail for queries. Then it can be accessed via SQL for data collection.