I think it is both a hobby and a luxury. But if it gets expensive I have a large garbage can in my yard and I can put the entire thing there. (after I give away the livestock of course) My reef is 45 years old and I spend very little on it. I know people spend thousands but that is because they want to, not have to. I let the LFS check my water for free, I feed clams for pennies a week along with live blackworms which cost about two bucks a week. I buy bulk supplements that cost pennies a year.
I keep my fish immune from diseases so I don't need any medications, hospital or quarantine tanks. I built my lighting system for practically free as well as my skimmer. I know many people can't or won't do that but those are a one time cost as my skimmer may be 20 years old, I forget. If you keep your fish healthy and immune you almost never have to buy any fish Some of mine are 25 years old so if it cost me $25.00- 25 years ago, that's a buck a year. Cheap by any standards. I also don't change much water, maybe 100 gallons a year so that cost about $100.00 a year. I have no controllers or dosers so I don't have to pay for that.
I really don't know why so many people spend so much. One year owning my boat in New York probably costs as much as 20 years of reefing. (and to store the boat in a marina is about $9,000 a year even if you don't use it)
My tank has gone over 90 degrees many times, nothing happened. Maybe that is because of the reverse undergravel filter I use. The power has also gone out numerous times in all those years, I deal with it in many ways.