What is Query cost ? what is it use?

Total cost to execute a query is 100%. In execution plan this 100% is divided across each part. For example to run a query (100%) with table scan (65%) and sort (35%).

have a look the link for detail

https://technet.microsoft.com/en-us/magazine/2007.11.sqlquery.aspx

http://stackoverflow.com/questions/564717/measuring-query-performance-execution-plan-query-cost-vs-time-taken

https://blogs.msdn.microsoft.com/sqlqueryprocessing/2006/10/11/whats-this-cost/ 

For query cost you can use SQL server profile and trace the queries you can get the desired cost for that queries whcih are getting fired from application .