You can spend a lot of extra money on shore excursions if you're not careful.
It's true that cruises have "all-inclusive" pricing, but that refers to the cruise itself, your cabin, basic entertainment, and meals on board the ship. If you want to take tours or go on adventures when you reach different ports of call, you'll pay extra (unless you want to go off and explore yourself which is a possibility in many areas).
So, how much do shore excursions cost?
It depends, of course, on where in the world you're cruising and also on what you want to do.
In the Caribbean, standard half-day shore excursions run $25-$35. In Europe and Alaska, they can cost twice that much. Adventure shore excursions cost more because groups are smaller and they cost more per person to operate. Expect to pay $50 and up for half-day adventure tours and $70-$150 for full-day excursions. Exotic tours, such as helicopter rides, cost $200 and up.