No random tests
Don't use test.expect()
In the tests it looks like you're supposed to return It's an empty message! if the input is an empty string?
It's an empty message!
But this isn't mentioned in the description and it's also not tested correctly.
Error with sample tests:
Traceback (most recent call last):
File "main.py", line 3, in <module>
Test.assertEquals(message_cost('Hi! I`m just arrived!'), '6 r. 30 k.')
AttributeError: module 'cw-2' has no attribute 'assertEquals'