Electrical consumption is a big factor. I have 1100 gallons of saltwater and my monthly bill for just the tanks is about $150-200, add salt for water change and that is about $70/month, food is another $20/month, bulbs add up to about $50/month, calcium reactor media, GFO, carbon is another $50/month. Gadgets go bad, so add another $50-75/month for them. Then there is the livestock which cost several hundred dollars a year until tank is stocked. My average cost for just my tanks is about $500/month.
Sounds insane but those numbers are totally true. Salt, Carbon, GFO, GADGETS and livestock. Yup 500 a month sounds right as long as you watch yourself because one can easily spend that much weekly as well, especially at the beginning with livestock. OP don't think about it just get the tank!
I am planning to do a reef on 1/2 and a fowlr on the other 1/2 water will mix but the non-reef friendly fish will not.
IME this is a mistake, pick one or the other and set up one big tank. If you want both then set up 2 seprate systems and use your reef water when you change your FOWLR. Large fish eat tremendous amounts of food and can produce a large amount of waste.
I'd say $100-$250 is definetly doable but like others have said it all depends on what you are trying to keep. I have 500 total gallons FOWLR and I spend $50 a month on salt doing weekly 50 gallon water changes (1/2 since I'm now doing bi-weekly), and $20-40 a month on food. Electric in AZ is killer in the summer when the AC run 24/7. Other than that I spend money on new livestock, and QT tank basics (salt, food, medication). The livestock is definetly the most costly part of my tank these days.