Is there any difference between these two sentences?
That dress cost less than i expected.
That dress cost less than i had expected.
Not really any difference, Njanja-- and thanks for all your good questions--
If the order of precedence of the two past events is open to confusion, and if they are clearly in a cause-effect relationship, then the past perfect should be used for the older event. If these conditions do not hold, then the simple past for both events is adequate.
Practically speaking, the past perfect is in a state of slow decline in the language, as there is little real use for it. Your example is a case in point: of course the expectation occurs prior to the realization, so the reader is not dismayed if you do not use the past perfect. Most people would say, and many would write, 'that dress cost less than I expected', and no one would be the wiser.