My 12 gallon ran close to $400 (including live rock, coral, sand, fish, water, etc), and the 55 gallon over $1000 (though the figure could be closer to $1500+ at this point, not totally sure) - initial equipment for the 55 gallon ran around $500.
I'm still planning a 180 gallon, which should be up by xmas '08, but just the equipment for that is estimated over $3000 so I'll probably piece it together as the budget allows.