For the set up, that was probably about $200, min. As far as monthly, probably nothing really. Except for electricity. Yearly? Food, water conditioner, possibly medications, I wouldn't expect to spend more than $50. But that is with a really nice set up. And then I do keep adding things that are for me more than the fish... Like more plants, different lights. But once you have the basics, depending on what you choose, they are pretty cheap.
Once you get into breeding or having more than just a couple (in divided tanks), the cost goes way up!
But for one fish, if you were to spend about $100 on a set up (cheaper if you get the tank/lights second hand), food, w/c, filter, heater, you would be super set for a really long time.
Not too much monthly...startup can be quite expensive though...and redecorating..but other than that a 10 dollar bottle of Prime and plant ferts. every couple of months and aquarium salt is all i need...unless a filter or heater breaks...or i see some nice plants i want...or a new fish...or decor..haha...
Mine costs $10 for rocks and $20 for silk plants. Then $20 for water conditioner per year, $10 on salts, $10 on pellets, $25 on frozen foods, and we'll say $30 on other meds. So that's... $130ish on my 5 boys. Plus $45 for each tank/filter/heater.