Reason for this is most probably that you are tryinng to return the same day. Returning the same day forces you to buy "Y" or "B" booking class, which are the most expensive.

There are cheaper fares available which have a minimum stay requirement (like 3 day min. stay or week end stay requirement.)

If you want to return on the same plane, I do not see how you can make it cheaper, unless you get an award ticket as suggested above.