Other pets may start out cheaper (if you adopt from humane society, ASPCA, or other adoption agency) ... but, most owners I know spend TONS of money on spoils. Cat cities

, beds, clothes, food isn't cheap, running water dishes, litter, chew toys, cages, etc.
Fish come at at cost up front. Especially if you do it right the first time. I don't know how big of a tank to move up to, but it has to be big enough I won't want to move up again after that! Tank, stand, filters, heaters, decor really add up at the start. But, once you are going, the fluorescent lights don't use much energy (usually all the lights on a large tank adds up to less than 100W, so usually at most 1 kWh a day for lighting). Heating isn't too bad either, even at 5W/gallon on a 300 gallon, that is 1 kWh, but they don't run constantly, so maybe 12 kWh a day. The filters from an Australian power company site says 10W, but even if you triple that, and have three running it isn't even 100W. So give it 3 kWh on the high side. So for a day, you have used 16 kWh to run a 300 gallon tank. As of 1/1/07, Chicago's electric was 0.11/kWh. So about $1.77 or 88 pence if I have the European money system right
Now, think about how much the average person spends on fuel a week putting around in a car. Or how much a drink costs at a corner store or drive-thru.
I don't go through food much either. He hasn't even ate one bag full yet, and he is done with the fast growing phase, though treats I have splurged on. So if I just fed pellets, maybe two bags a year ... the most expensive stuff I have found is Bio-Gold+ at $20 bag ... $40 (20 quid) a year is so much cheaper than feeding most cats and dogs.
For tank maintenance ... a bucket, siphon, and python was a one time buy, and not too much. I buy API products StressCoat, MelaFix, and PimaFix (the last two when he gets clumsy and hurts himself). The direct API product is fine for smaller tanks, but API also makes these in a stronger dose under their PondCare label. Same product, but last MUCH longer. 15 mL for 60 gallons on the StressCoat dechlorinator. 5 mL for 50 gallons for PimaFix and I believe the same for MelaFix (maybe it is 10 mL). So, it goes a long way and they are only a few bucks more.
Bio-media almost never needs to be changed. Filter pads can be rinsed in tank water quite a few times before needing to be tossed.
So really, Spread out over the life of a fish (Oscar 10-20 years) ... Fish are much cheaper than any other animal, really.
At least I have told myself that