First of all thank you very much for using the WP travel plugin.
Currently, the plugin will display “0” price as you have mentioned. But to add the message as you have suggested requires code customization and as much as we would love to help you with this, we are unable to do so as it requires higher code customizatin.
If you require then you can consider hiring the professionals for this.
To hire a developer please refer to below given link:
In settings there is an option to show text if price is empty or zero.
But that is not working.
Text is not shown when you do not enter a value in the price field.
When you trie to fill 0 in the price field then you get message that is not allowed.
For multiple priving options it is even not possible to leave the price field empty.