Greatest cost is the initial outlay for equipment. This is where research and a willingness to buy used will help. A good light fixture is worth the money.
Once you get into upkeep, no cost is crippling but there is a steady, low-level expense that you need to be aware of:
- salt - most people do 5-10% weekly water changes; on a bigger tank you might be able to go a month but would need to do a bigger change. So imagine on a 200g tank, if you did a 20% monthly change, you'd need 40g of saltwater. That's about $40 worth of salt per month, or $480/yr.
- water - buying RODI at the store or buying an RODI unit - the more water you make the more frequently you will have to change your filters and resin
- supplements - mainly, alk/calc/mag. There are budget options for this (the BRS mix-your-own supplements) but when you're just learning how to keep a tank stable, it's good to go with quality, dependable, proven brands like the B-Ionic supplements. If you are dosing 200mg+ per day because you have a 200g tank, you can easily spend $100 a month on this. - bulbs - depending on the number and type of bulbs you have - PCs should be changed every 6-9mos and T5s/metal halides every 9-12mos. A big tank like a 200+g would likely require 3-4 metal halides; depending on the bulbs you go with, this could cost $100-250.
I have a Solana 34g with a 150w metal halide, Tunze 9002 skimmer, and a Tunze Osmolator auto top-off. I also have a Marine Color dosing pump, although most people wouldn't consider a dosing pump to be must-have equipment. I have always bought used and resold as I've upgraded; I started with a $60 used BC14, next tank was an $80 used NC24, and finally I got this $200 used Solana with stand. My skimmer and ATO I bought used. I've never made back 100% on selling equipment, but I since I buy used I don't loose a lot on the resell price. We have four tanks in the house (I also have a pico, $20 used, and my boyfriend has a 20g and a 30g), so it's a little difficult for me to say exactly what I spend on upkeep for that one tank, but I have a nice variety of softies, LPS and SPS as well as fish, and it's relatively easy to keep up.
Maybe someone who keeps a big tank could chime in with real cost estimates...