Somewhere north of $5,000 for my 25 gallon AIO. I have a decent amount of automation built in and I've had a fair amount of issues like Bryopsis, cyano, etc. So I'd say at least $1,000 to additives and testing alone.
Ultimately I invested a lot up front so that when I upgrade in like 4 years, I can keep all of my equipment and literally just need a better light and more sand/rock.
Also, I should note that I have a years supply of testing reagents, RODI filters, filter media (carbon, GFO), additives, and food on hand