Believe there is no cost. This the the website, https://www.nps.gov/nisi/index.htm it is a wealth of information. My time there this past May was for a phase 3 archaeological project within the Star Fort but there is much more to see aside from the Fort itself.
It's free to visit, 1103 SC 248, easy to find
It's free to visit the site.
The address is 1103 sc- 248 it was free to get in last time I visited
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