Do you have live rock? Anything else for water movement? What's your lighting like?
The maintenance for a reef isn't much more than for a FO. There are a few supplements that people use--calcium being the biggie. Feeding filter feeders requires daily or every other day dosing with appropriate foods, but that's about it. The lighting is a big deal for most corals, though. They contain photosynthetic critters in their cells, and without these, they can die. Like keeping aquatic plants, this means paying attention to not just the amount of light provided, but also the quality and color of that light. 3 watts per gallon of PC, HO, VHO lighting will be enough for some things, like some polyps and mushrooms. More--up to 10+ WPG-- is used, depending on the tank depth, the corals you want to keep, etc.
Live rock and sand is usually the recommended filtration--you get critters to keep it clean, it hosts bacteria that deal with nitrogen wastes. The nitrate reducing ability of 3-4 inches of sand really helps out. Most people run a protien skimmer-this helps remove nitrogen wastes as well as pulling out toxins from certain corals.
For costs--probably about the same, depending on what's in your FO. Feeding a lion and a trigger costs about the same as providing for the corals--once you've paid for the equipment, of course!
