Sassycruiseher, if you make private shore excursions with the concierge service before you leave on the cruise, there is a $100 charge per port. You can, however, make private shore arrangements with the tour office when you are on the ship without a charge.
I have found that at times it is better to do it with the tour office on the ship instead of with a pivate tour company just incase for some reason a port is canceled. Also if you do it with the tour office they are responsible if your tour is delayed because of traffic or some other unforseen reason.