Expression of interest is free... It costs to get to it though... let me explain:
You get Expression of interest(EOI) when you apply to SkillSelect with your notarized references and diploma and report form agency (in my case ACS)...
EOI is a farther step in the process of applying to get visa... where once you know (hope) you have sufficient points from your assessor (ACS), then you go to SkillSelect and give them you IELTS reference, ACS reference, all the other point scoring details (this is you showing EOI), and then you get your points report based on what you have submitted (see bellow)
you then wait for email that asks you to apply for visa where you have to submit everything again...
And this is where it will actually will COST you... 2k+ and they can give you different points and you cannot appeal...