Depends greatly on what your trying to do. Do you want just fish or do you want coral too? Fish only is much cheaper and you can use filter and everything else from a fresh water tank with no extra equipment. Set up is always the expensive part but keeping it running isnt bad just electricty and then buying salt.
If however your doing coral its going to be much more expensive. Youll need a better filtration system, a protein skimmer and far stronger lights. Depending on tank size and thus lighting requirements, this could considerably raise your monthly electric bill.
Sorry I didnt give any real figures, but its hard to say without knowing really what you want to do and how big of a tank.