I realized that it was enough to have a string containing both the component name and the price. In this way in the admin page for adding a new configuration (that shows the components list too, due to edit_inline=True) will be shown a combobox with, as said, the string 'comp name -- price' obtained via __repr__.
Initially instead I though that a tuple could keep the two values separated, so that they could be used in different contexts. For example, and it was my initial question, to have in the admin a listbox 'Component' filled with the name, and a dummy-display-box 'Price' filled with the price. Sorry for the confusion, I'm new to Django and I'm still trying to figure how the things work together.
