about how much does a small generater cost?
Depends on how quiet and reliable they need to be, how clean the generated electricity needs to be, and how many watts you need to run your tank. If you are just powering pumps and heaters, then a small generator can be a little as a few hundred dollars. If you need to power electronics (such as an aquarium controller, a vortech pump driver, a computer, or similar) then you need a generator with an inverter (provides "cleaner" power) which will cost you several hundred dollars more. If noise is a concern, the inverter generators (such as the Honda and Yamaha brands) are also the quieter generators - running at or below 60 decibels. These are the portable types that most people buy for RV's since they are relatively quiet.
At Home Depot, you can get a Triron Small Portable 1200 Watt Surge Generator for as little as $150. I have no idea how well that generator works. Conversely, a Honda EU1000iA 1000w Inverter Generator goes for $860 MSRP. The Honda includes extras like an electric start, low oil protection, and a variable speed throttle for fuel economy. I have recently found many of the Honda Inverter models on my local craigslist going for less than half price.
There are a few vendors who can convert Honda or Yamaha inverter generators to bi-fuel or tri-fuel (gas, propane and/or natural gas), so you could hook it up to a tank or a clean burning NG hookup. That conversion will add a couple hundred to the price of those generators.
Somewhere in between those extremes you will find larger (non-inverter) generators that will give you even more wattage. 2000w, 3000w, 4000w generators can be had for $400-$600. They are as loud and heavy as all get out, but they may be a good compromise since they offer much more fuel capacity and wattage than the ones previously mentioned.