Using Rank for associating weightage to a story

I don't think rank nor complexity should be used as weightage. Story points tell the complexity/how hard it is to implement it/how long it will take in an abstract way. The rank is used to order the work.

The business value is just an indicator to hint for ranking. In general you want to address the highest business value first. However, if something has a medium business value but a very low complexity, you might decide to address this earlier than the hugely complex story with high business value, just because of the resources available.

I would suggest to create a new attribute to store information about the business value. I would probably use an enumeration like being used in the story points. You can see how attributes can be added for example here: https://jazz.net/library/article/1093 lab 3 and following labs.

For weightage, you may want to use the complexity attribute.  By default its the Story Points. 

For the Rank, if you are on v403 you can take advantage of the new Ranking feature:
http://pic.dhe.ibm.com/infocenter/clmhelp/v4r0m3/index.jsp?topic=%2Fcom.ibm.team.apt.doc%2Ftopics%2Ft_rank_work_items_web.html