ferry to St. Petersburg

I am traveling to Finland to visit relatives I have never met for two weeks in late May and early June and will likely travel with them a lot but I want to go to St. Petersburg and Estonia. I know the schedule of the ferry to St. Petersburg but I don't know how much it costs. Can't seem to get that information easily. I also know I have 72 hours visa free. I will likely be there by myself. Whether I go to St. Petersburg will be determined in part by the cost. If it is too much I will skip it and go to Estonia for the nights. I hear it is very cool. Anyone have that information? Thanks