I try not to think too much of the "monthly" costs or "long term" costs of maintaining my tank(s)...
It's kind of like having kids...If I'd have known it was going to take $500,000+ each to raise them (over the course of their life)...I don't think I would of had 4 of them....
Having a tank is kinda like having a kid...First you have the high initial costs of bringing them home from the LFS (or hospital) then It lightens a while, while you feed and raise them, then there are the "upgrade" costs as they grow (new cloths, better skimmer, a bicycle, maybe a bigger pump) and the unexpected costs (broken arms, sickness, Ick or a bad transformer) and then your back to feeding and cleaning up after them...
And of course there are the "expected" regular maintenance costs of school supplies, new bulbs, more new cloths, and an additional "specimen" or two...
Then just about the time you get your head above water (so to speak) they hit you with the big expences..Collage, Car, Bigger tank, Closed loop , Chiller...It just goes on and on...
At least with the Kids they'll eventually move out and get married...the tank then becomes more like the wife..your stuck with it for life...
I gotta stop now...I'm depressing myself...