There's no fee to walk around, but there is if you want to go inside the museum... which I'd highly recommend. I'd also suggest buying your tickets to the museum online and save time in the lines, they tend to be long.
You don't have to pay to see the memorial itself, however you do have to pay to enter the museum. Museum tickets can be purchased online or in person. The ticket prices vary depending on age (adult, adolescent, and child). There is also a discount for veterans.
You can order online. We paid $26 for 2 tickets
No. 9/11 Memorial is free.
Tickets are based on time slots, though, so you might want to consider buying ahead of time if you're on a time crunch
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