Please note, this configuration is possible at the Business Medium or higher versions.

Description
If you are need to add the additional costs, like tax or some other additional charges (cleaning,breakfast, excursion...), or just increase the cost depend from the visitor number selection at the selectbox, which is depend from the selection of the selectboxes or checkboxes in the booking form, so then you can make this configuration in the Business Medium or higher versions.

Configuration
1) You can insert into the Booking form, at the Booking > Settings > Fields page some checkbox, like this:

Tax: [checkbox* my_tax ""]

If you are need to have this checkbox hidden and checked by default in this way:

<div style="display:none;">[checkbox my_cleaning default:on ""]</div>

If you are need to have this checkbox as required and checked by default so use it in this way:

Cleaning service: [checkbox* my_cleaning default:on ""]

For the selectbox, its can look like this:

Visitors [select visitors "1" "2" "3" "4"]

Please read more about the booking form fields configuration here

2) Then at the WP Booking Calendar > Prices > Form Options Costs page at the advanced cost management section for the option my_tax or/and visitors, set some additional cost.

For example, for the tax 6% you need to set value 106% there.

Or if you want to add the fixed cost value, you can set this value to this "my_cleaning" option:

my_cleaning = 50

or for the visitors selections you can leave this configuration:

visitors:
1 = 100%
2 = 200%
3 = 300%
4 = 400%