No one has mentioned also that if you start and the "bug" bites you, you'll find yourself direct-depositing your paycheck to Amazon and BRS after a while anyway.
But that bug bite hurts so good. Until your wife sees the bill.
I'm about to say something you may not want to hear:
If you're concerned about monthly expenses, this isn't the hobby for you.
Building a reef or FOWLR tank is not a budget proposition, either initially or monthly. Minimizing monthly expenses via LED's, good DC pumps is always possible, but initial costs can be very expensive.
You will need ro/di for evaporation, water changes.
Food? Well, you can't scrimp on food, either. Fish and corals need food. Some need lots. Maybe you can make your own?
Ultimately, if you're on a very tight budget, IMO this isn't the hobby for it.
I think the best money saving advice one can give to a new reefer is not to skimp on initial setup. If you try to skimp you will end up replacing things further down the track either because it is rubbish and broke or it is rubbish and doesn't do the job effectively.
Buy quality, buy once. The poor man pays twice.
Just keep it simple... This hobby can definitely be overwhelming at first. Buy this, buy that etc.. The more experience you gather you'll start to realize that you don't really need this or that to accomplish the same results.. It's kind of like chess.. Why win in 10 moves when it can be done in 5... GL.