We found that most excursions were still available at 75 days out. There was one excursion we couldn't get nine spots for, so I asked for 8, 7, 6, etc. It finally approved 5, so I took those. (it doesn't tell you how many spots are open, only whether or not the number you asked for are available.) Then, over the next few days, I kept checking back, this time asking for 1 spot at a time. Within a few days I had the four more I wanted. Remember, you don't commit to these excursions until three days before your cruise, so lots of people reserve them initially and then back out later. Likewise, if I couldn't have gotten 9 spots, I would have had the option to drop all or some of my spots closer to the trip.
As for the costs, they vary widely $50, $100, $200 per person. And remember there are several islands. You can do as many or as few as you want/afford. You can always just get off the ship and look around. Some ports are better for that than others. You can also book excursions not through Disney. Some people love that. Others don't. Search for threads talking about excursions on your itinerary and you will find lots of feedback and experiences.