Hi! The big expenditure in this hobby really is buying the equipment to get started. Once you've bought the tank, a filter, stand, heater, and fish the hobby really is cheap. Once your tank is cycled you really don't need to test the water very often if at all. Flake food is cheap and goes a long way if you have regular smallish sized fish. If you do the proper maintenance (eg water changes) then your fish should be healthy and you shouldn't need to buy medicines often or ever.
As for the cost of running the equipment, the filters use next to no electricity, the heater will draw some depending on the wattage (eg, mine is 300 watts), but it isn't on constantly. So when it is on it's like having three 100 watt light bulbs on. Same goes for the lights; if you don't have some fancy set up your lights are probably less than 100 watts.
Chances are if you look around your house you'll find lights on when they don't need to be, or old refrigerators that are terribly energy inefficient, someone leaving the tv on when no one's watching, computers etc. etc. Really in the grand scheme of things, i wouldn't worry too much about one aquarium as being a big energy draw. If you want to do a reef or high tech planted tech set-up with special lighting, CO2 injection, or some really big tank then the costs are going to go up. Just take a look at the wattage rating of all your aquarium equipment and compare it to other appliances in your home.
