How much do you spend on all of your fish each year?
I've probably spent about $300 on aquarium stuff in the last few months not counting
the aquariums themselves (I had a small one then upgraded) but
most of that is startup costs for stuff that I won't need to replace constantly - fish, large bottles of chemicals that get used slowly, filter units, heaters, nets, gravel vacuums, etc. Also, you should understand that I'm a middle aged professional with no spouse or children, so I have the money to spend if I want to try something (like a heater), decide I don't like it, and just buy another one. So, I know I could have done it more cheaply if I'd wanted to.
Now that I've got things pretty well set up and I have basically what I need in terms of chemicals etc, I'm expecting the maintenance costs to be around $5/month, that consisting of food and occasional replacement chemicals. (Understand, I have four bettas and also I have a larger tank with 8 fish and a shrimp in it. If it was just the bettas I'd be thinking more like $1 to $2/month.) I'm not counting the electricity to run the tanks, I am not interested in calculating the cost of that. I'm also not counting changing decorations when I feel like it.