Worth the money, beware of the strenuous climb and heights.
Sitting Bull Crystal Caverns was a very interesting cave. The tour guide was very educated as well as humorous. The stories and legends of the cave as well as how the cave formed was very interesting. Getting up and down was a pretty steep climb. I wouldn't recommend it for young children. Also, I wouldn't recommend it for the elderly or anyone who has trouble with stairs and climbing. One more thing to think about is that the cave does get cold, much colder than outside, so dress appropriately. The handrails are pretty cold and slippery, so you need to be careful.
Overall, it was very beautiful (the crystals were awesome) and definitely a worthwhile experience for a reasonable price (I believe it was $8.50), just make sure that you are capable and can handle the heights and steps.