It is true what all of these guys are saying, its not easy to make a lot of money but you can offset the cost of the hobby
here is what I have done in the past to off set my costs in the hobby, buy a large colony of any coral, i choose to do sps when i do this. Make it something nice and rare, doesnt have to be the rarest. Buy it from an online store so you get the best possible price even when shipped, put it in your frag or qt tank for a few days to make sure it is healthy, then frag it, cut it into as many 1-2 inch frags as you can get, mount them on plugs, and then put them back in your frag tank, wait a few days to see which ones take and which ones need more time, call up your lfs and say that you have x number of frags of x type of coral, bargain with them on a price, start high, sometimes you get lucky, ask for 10 a frag, if you buy a colony at 60 bucks, pay 25 for shipping, so lets say a total of 85 bucks then sell 15 frags at 10 bucks a peice you have made about 65 dollars. obviously by ordering more corals at a time and fragging all of them you can come out way ahead in this game, perfect for store credit, supplies and trading. save one of the frags for yourself and you've made out pretty good.