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.