...the Scavi tour?
You should be able to get to the Scavi tour with your ticket without paying extra. Unless you want to enter to see other things such as The Vatican Museums and Sistine Chapel which cost approximately 15 Euros and St. Peter's Dome is approximately 6 Euros. St. Peter's Basilica, St. Peter's Square are free. The admission to The Vatican Museums and Sistine Chapel are combined.