Visiting the park and grounds is free. Cavern tours are $17.50 for adults, $12.75 for children 3-12. The cave is 68° and humid year round.
The caverns have a constant temperature of 65 throughout the year. For pricing check their website.
Their website would give the best pricing info but I can tell you that the caverns are pretty cold. A normal coat should be fine, no gloves or hat.
https://visitlonghorncavern.com/
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