when i lived in a house with 3 55 gallons, our water bill was never more than $20 a month.
however, different parts of the country have different water prices, and during a drought (which we never have here) the prices could easily skyrocket. i know theres a city just a 2 hour drive from here who's water bill is ridiculous and would probably make anyone think twice about adding a fishtank.
You can always take navy showers if you need to make up for the water you use on your fish tank.