If you try to edit additional cost at the Booking > Resources > Advanced cost page and during saving, all your changes removed back to default values "100%", then please check these steps:
1) Please be sure, if you are using custom booking forms, that the length of custom booking form name not longer than 30 symbols. If its longer, then you need to recreate your custom booking forms.
For doing that, please open the Booking > Settings > Fields page. Select your custom booking form.
Then copy content of "booking form" and "content of booking fields data" form to some TXT file.
Click on "Delete" button in the toolbar to delete this custom form.
Then create new custom form, where name must be not longer than 30 symbols. Paste in the forms your saved data and save changes.
If you was inserting into the posts or pages, booking shortcodes with previous custom form, you need to reinsert them.
Now you can test it.
2) Please be sure that you do not use in the names and values of checkboxes and selectboxes in your booking form some non standard symbols, like umlauts, or % or ' ,etc...
That's why please open the Booking > Settings > Fields page and for the each booking form (if you are having several custom booking forms), please recheck (re-create) shortcodes by using only standard symbols in the names and values.