There are no upfront costs with Booking.com. It's free to list and then they charge hosts a 15% booking fee every time a booking is completed on their platform.

As part of your agreement with Booking.com, you’ll pay commission on every booking made through their website by guests who completed their stay at your property. 

You may also pay a commission or relocation fee if you decide to cancel a confirmed booking.
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You can learn more about AirBnB's cancellation policies by clicking here.