Based on the existing 'Include / Exclude Contacts in a Group / Tag' custom search, this produces an exportable list of contribution totals & counts. The count shows the number of seperate donations that make up the total. The search has been tested on CiviCRM 2.1.4 and 2.2.2, running on Joomla 1.5.9.
How to use: Put the code below into a file called GroupTagContributions.php in CRM/Contact/Form/Search/Custom.
Custom Search Path: CRM_Contact_Form_Search_Custom_GroupTagContributions