Partially closing the AC duct/vent to the room with the tank (really for bigger tanks and fish rooms) in the summer so that the room is a bit hotter. Just don't make it uncomfortable. You may also want to look into getting a top for your tank and minimizing heat loss even more by lowering spray bars into the water, raising your water level so that any HOB outputs are not creating a "waterfall".
I do agree with everyone that a 55 gallon tank (even at that temp.) could not cause such a spike in your bill. Filters are pretty irrelevant unless you're running a pressure biased pump (they eat electricity) or a very, very large flow biased pump. Lighting should not be too high unless it is a planted tank and you need bright lights on 14 hours a day. So it all comes down to heaters, your's being set a bit high, it could draw a decent amount of power, but still not $100 worth.
To raise your bill (assuming $0.10 per KwH) it would take roughly 1000 watts. That is more than your entire tank, with heaters running constantly, could ever draw. My guess is that your bill got raised (ours has been about 3 times in the last year), they added on some BS surcharge, you are just using more power (e.g. new appliances, new computer, too much MFK, etc.), or you have a faulty appliance or wiring fault. While the latter is highly unlikely to cause this type of problem, a compressor (fridge or AC) that is going bad could indeed raise the bill.