For the full tour it's $30.00 per person and you have to go on line to make reservations and it's very worth it!:)
You should visit the website www.stetsonmansion.com for details. Pricing varies depending on the type of tour and time of year.
Why don't you call the museum?
You should speak to them directly, visit their website. I would image for a tour of 15 you probably are big enough to be your own group and might be able to snag a discount.
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