The basic reason why you can not balance the system to get the same temperatures is the air distribution system.

If you have a seperate main duct feeding the 2nd story you may be able to have a zoning system put in. This way you would have a thermostat on each level.

If you have a situation where the AC doesn't run very often you can easily get 5* difference between floors when the fan is off. Rebalancing the registers and ductwork flows can help a little, you will wind up with the opposite temp difference when the AC is on, usually, though. If you have open staircases between the floors it is going have a temp difference from the heat rising. Running the fan contiuously works well, but will cost a bit. Make sure the AC blower has a high efficiency motor on. Ceiling fans and strategically placed box or oscillating fans also help.

We have a wide open 3 level house and put a box fan at the base of the lower level stairs, blowing up into the staircase and there is a ceiling fan in the top level above the staircases. With the two fans on the temp stays very good in those rooms, but still varies in the other areas of the house. If we have a party or such, we put away the stand alone fans and run the furnace/AC fan contiuously, which works great.

You can calculate just how much it costs with some simple maths, provided you understand the units of electric and what you pay per Kwh. It should say on the bill what a Kwh costs you, although sometimes you'll find two or more different prices. I pay around £0.13 for the first 222 kwh and it then drops down to about £0.06. If you're allways going over in to the second pricing scale then you should assume the extra added to your bill from a new appliance will be at the lower price.

As for a Kwh, if you aren't allready aware it's a standard measurement of one-thousand watt drawn over one hour. So, a 1000w electric heater uses exactly a Kwh every hour, while a smaller appliance that draws 500w uses a Kwh every two hours. And, something really small like a 100w light bulb only uses one Kwh for every 10 hours of usage. The fan should have the wattage printed on it somewhere, and by using this you can calculate how many watts it would use over 24 hours, then multiply by the cost of electric to give a daily figure, which can also be multiplied by 30 to give an average of monthly usage.

For example, if I ran the 400w motor on my central air, which I should add is very old (1968) and possibly eats more electric than a modern appliance...

A 400w motor times 24 equals 9600, which is exactly 9.6 kwh each day.

The cost of electric where I live is .13, that multiplied by 9.6 equals £1.24

Asumming an average of 30 days in a month, the aproximate addition to my electric bill would be 1.24 x 30 = 37.2

If you really want to know the yearly cost multiply it by the aproximate number of months you run the central air, but be prepaired to pull a very bad face when you realise just how much it really is costing you. I calculate that the two months a year that I use the air conditioning would cost me an extra £74.4 if I let the fan run all the time. Not too shocking, but if I lived in a hot climate where I used it for five months a year then the cost would grow to almost £200.

You can even calculate the carbon emissions from it, if you're curious. I suggest everyone try it just once, if only to have the "Wow, that much?!" moment that they allways seem to. Of course it varies from plant to plant, and different fuels, but the average release of carbon per kwh is 0.46 kg, that is taking in to account all the different types of plants such as coal, natural gas and even nuclear to reach an averaged number. So, for one day of use that 400w fan would release around 4.42 kg of carbon, which over a month would be aproximately 132.6 kg.

My church does this, year round. They leave the fan on in their HVAC system 24/7 so it doesnt get "stuffy". I have tried to explain many times if it's circulation they want, use ceiling fans . . . that having been said many newer HVAC systems have fans that use VERY ERVY little power . . . in the case of the new furnace my mom had installed, less than even one ceiling fan . . . for what it's worth.