How much does it cost? It depends on what is included, but not as much as you might think. Almost certainly less than your hotel room, for example. If you are a student backpacking through town, a custom tour might be out of your price range, but if a cab ride to the airport won't break your bank, then neither will I.
When we get down to specific figures, you will probably want to convert them into your home currency. In case you don't already have a currency converter you like, here is one I've found to work well.