Two Methods:
1) Add a 'Package' reservation and put in a simple description and then the price. This will allow you can invoice it.
Packages is a sub category of the 'Tours' reservation category.
2) Manually type in the price information into the proposal/client itinerary.
There is the general 'Comments' text box or the Master itinerary description box at the beginning of the proposal/client itinerary.
You can also scroll down, there is 'Details' box where you can add any section so that you want. Each section you create has a title and body text. So you could add 'Price" for example.