I put $2000-3000. My current filters on my 3 active tanks alone were $750, and at one time I had 7 tanks. I spend a lot on fish, too, let them grow and trade them back to the fish store when they get too big, losing at least 50% of my money each time. I have all my tanks also hooked up to their own battery backups so if the power goes out they keep ticking.
But this is probably nothing compared to what I have planned. I want a public aquarium standard tank built with 4 sides (3 sides and bottom) concrete (yes, I know standard concrete isn't waterproof) with one side glass or acylic, aiming at about 3000 gallons. I want it to drain out to a manually built filtration system I intend to build out of my current store bought tanks and will pump back in using a fountain pump. Contrary to most big fish tanks though I am not really interested in many big fish, I'd prefer to keep maybe 1 centerpiece fish and then a lot of smaller ones. Right now one of my 75's is a species tank of small fish and I really, really enjoy that more than having one large fish like I did previously... But, the hobby always changes... That's the joy of having a fish store that takes fish back. Love exploring the hobby.