Is it possible to format a field cb_price to add at the end of the field the string : ‘eur’ ?

No, not without a new CB plugin to append it to the value or better yet the display. You could also try using regex validation and require users to input the "eur" themselves.

I also have another problem with the field zipcode : I add /^[0-9]*$/ to show only digits.

This only applies to input, not display. Perhaps an integer field is more appropriate?

But when I use the search in the userlist I can type letters ! How could I avoid the user writing letters ?

You can't, it's a text field. Users can type anything they want when searching userlists. The validation applies to actually adding a value to the field.