If you have a lighter stocked 20-30 gallon with easier to keep corals, it can be very cheap. Tanks are cheap, heaters are cheap, you won't need an expensive light, you could just go with a simple hang on the back filter for mechanical filtration, a couple of cheaper powerheads, buy a little used sand, buy some used live rock and/or buy dry rock, pick up cheap frags to grow out and less expensive fish, buy cheap salt, hydrometer, scraper for the glass, some type of stand to put the tank on and you are done. You could probably find a cheap fresh water setup on kijiji and just add of couple of things and be set. If you don't put a lot of stock in a setup on a smaller tank, you can easily get away with no skimmer.
Here is a set up for $50.
http://alberta.kijiji.ca/c-pets-acce...AdIdZ261147119
A light set up would probably run you $100 used. You would want 20 pounds of rock around, so that would be between $40 to $60, a hydrometer is $10, sand would probably be maximum $20 used, a couple of powerheads used would be around $40, salt would be around $45 and a scraper would be around $10. That's $325 to get started.
For maintenance, you want to do 10% water change a week or 20% every other week. For a 20 gallon tank, that's 104 gallons/year. A bucket of Instant Ocean is around $45 and that does 160 gallons, so will last you more than a year. Your other weekly fee would be food, but you can pick up a container of it for $10-$15, and some frozen for around $10, and that will last you a long time. Your electrical bill will go up, but not very much on a simple set up. You will need to change out bulbs every 6 months to 1 year, depending on the type of bulb.
The other major maintenance costs are additives to get your water very similar to ocean water for harder to keep corals. If you keep it simple, I wouldn't even bother making sure your water is perfect.