Frugal and Energy Efficient Living

"But if I can't turn it off and must turn on the AC, it sounds as if the powered attic ventilator might actually cost me MORE $ then. That's the only time I'm concerned about and is the main focus of my question."

Of course you can turn it off. Any contractor who installed a whole house attic fan without providing a master control switch should be reported to the licensing board as incompetent. I would not even have a thermostat, because that is reactive, and a whole house fan should be used proactively. It should be run when the air is cool to cool off the structure of the house and then shut off when the outside air is hot. Once the house has heated up again, turn it back on to move the air. No computer or thermostat can do that for you, since you do it based on how the air feels.

When airconditioning season begins just leave it shut off, but using it will certainly shorten the length of time that you need to use air conditioning. Of course you do not use the fan when the air conditioner is in use, and it has automatic gravity-actuated shutters to prevent air flow when the fan is not running.

Don't let the contractor sell you any fancy, computerized control systems for it. All you need is an on/off switch and your own brain. A little experimentation and experience will teach you when to use the switch.