No charge for the park entrance. You have to buy ticket for the mansion tour.
The mansion tour is $8. $20 for the tour and Billings Farm & Museum
It is free to walk the trails of the National park and to walk around the property. It does, however cost money to take a tour. For adults 16-61 its $8, for children 15 and under it's free, and for seniors 62 and above it's only $4. Billings farm has separate prices, as you must pay an entrance fee to see the whole farm, exhibit, ect. However, there is a combo ticket that saves you a couple dollars if you wanted to see the Billings farm, then head over to the National Park for a mansion tour.
How much is the tour ticket?
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