We are going in July. There are a total of 25 shore excursions in Rome. 3 of those fit what you might be looking for. Express train is $89, bus transportation from port to St Peters Square is $66 and transportation from port to Hop on Hop off bus is $119
we are doing a pricey excursion as there are 17 of us and I have been before where there can be long que lines we want to avoid.