Cost of big tanks

The main costs will be, Electricity, Water and Food.

A lot will depend on what you pay for electricity which can vary quite a bit. In some areas of the country I have heard rates can be as low at $0.04 per KWHr, but other places the cost can be as high as $0.40 per KWHr.
The cost of water also varies.

Below is a typical example of the monthly cost to run a 300G, assuming the national Average of $0.11 per KWHr.

Two FX6 filters- $6.82/month
Lighting (200W- on for 8 hours per day) $5.28/month
600W heaters- (on for 3 hours per day) $2.97/month

Total Monthly cost for electricity = $15.07

The amount will obviously vary depending on the equipment you are running, how often the heaters come on as a result of the air temps where the tank is setup, etc.

I pay about $5 for 800 gallons of water, so if you do a 50% water change every week, that's $3.75 in water per month.

The cost of food is going to depend on the dietary needs of the fish you keep, but let's say $10 per month.

So, all in you are looking at roughly $30 per month. Electricity is really expensive where I live, so the cost would be closer to $50 per month if I used the rates that I pay for power.

my 6 tanks( 350 gallons),runs $50 in the winter$ 15 in the summer ,this is electricity only

I live in las vegas and were in a bad drought and our summers reach close to 120 degrees, for 3 110g and a 270g were paying between 175-250 a month in summer and as lost as 100 in winter here monthly. For us our power comes from the hoover dam which is expensive and our monthly water bill is over a 100 a month in the dry season.

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Depends on your filter setup and lighting. If your running a sump with a pump + canisters. Or just canisters or just a sump. Will you be running LED or Fluorescent or Metal Halids. Some people like to run big Jacuzzi type pumps in there sump some run smaller pump. Is it a fish only or live plants or Reef will determine your lighting needs. It can vary depending on what you plan to keep in the 300.

Please..keep in mind....although everybody's situation is different...in GENERAL..on AVERAGE...if you live in a place where electricity or water is VERY high, like out west, you are very likely to be making a higher salary to keep up with these high costs. It's expensive to run a tank in Los Angeles, but on the other hand, in LA you can get a used 500 Gallon acrylic tank, too. You can get AWESOME fish for cheap--PU only. If I want a 500 Gallon Acrylic and cool fish where I live, I pay top dollar for a new custom tank and FedEx to ship fish in at $90 a pop. But, I pay 8 cents a KWatt.hour for electricity and water is so cheap I couldn't tell you what they charge for it. It all evens out in a free market economy.