How to use logical formula while preparing cash flow?

using Script logic will be the best way. But if we  want to go with simple dimension logic...then 

>[[NETAR]]-(][NETAR]],[[TIME]].PrevMember)     

(no square brackets for Prevmember as it is MDX command, not dimension/dimension member) 

can be used to get the difference between this month and last month. 

Previous month returns previous member in the level that contains a specific member.