When my wife first said she wanted a tank, we looked at a 46g bowfront... After visiting the LFS, she decided she wanted a 120g... So, we purchased the tank, a stand, and a matching canopy to go with it.
I researched lights (LED) for a couple months and ended up buying a set at around $1700...
Spent more time researching products and reading reviews - in fact I have a Word document that is nothing more than a list of products I'm purchasing, a price, and the website I found it cheapest at.
So, here it is, nearly 5 months later, and I haven't put the first drop of water in the tank yet... I've still got another roughly $1200 to spend before I can put water in the tank and begin spending more cash to stock it...
That will bring my total spent to get the tank up to about $5k for a 125g with a 40g DIY sump... and remember - no stock in the tank for that cost.
So, when it comes down to the monthly cost... It's only money right?
If you're talking a 200g tank with a 100g sump - I'd think that ~$150 - $200 is a reasonable cost - that's an average that includes food, elec, water, other misc equipment.