This has finally been resolved in SSRS 2008 R2 "Calculating Aggregates of Aggregates" http://technet.microsoft.com/en-us/library/ms170438(SQL.105).aspx

For the time being you could probably setup a Function in the Code section and then pass in your values. Something along the lines of this example http://stackoverflow.com/questions/213002/calculating-grand-totals-from-group-totals-in-reporting-services.