Just to index this in some way, here it is adjusted for 100 miles, at the average cost of residential electricity:
Prius Plug-In (100 miles): $6.12
Standard Prius (estimate, 100 mi): $6.98
Difference per 100 miles: $0.86
Difference per 10,000 miles: $86
Difference per 100,000 miles: $860
Full review below
2012 Toyota Prius Plug-In: By The Numbers, Would It Work For You?