I read a little while back that idling a modern car typically uses 0.2 gallons/hour. So, if you consider that the minimum, then you're looking at at least $0.70/kwh. It is possible to achieve full draw on an alternator at idle, so I think it is reasonable to assume you could get 700w out of the car at idle. There would be a nearly negligible increase of load on the engine. $0.70 / 700w = at least $1.00/kwh.
If you upgraded your alternator to allow for more load, then you might need to also increase the default idle speed to ensure the engine does not stall, which would mean more fuel usage.
EDIT: Now if you're talking about running it on a car that is moving, I'd say the electricity is virtually free.
Extending on that, you don't want to run your car at idle for power generation - you want to run it at the peak torque RPM most probably, very often something like 1800RPM-2200RPM. Larger the engine, lower this is.
Also, the typical car here consumes more like 1.5L/hr, so more like 0.35gallons/hour, and at 1800RPM you are probably looking at 0.5-0.6gallons/hour.
Car engines are not designed for low load constant speed - you need to increase the load ie. alternator capacity and utilization dramatically to get the best out of the engine, even at 1800RPM low load you are running perhaps 2% efficiency, while a traditional, older car engine could reach 20%.
for thought experiment, let's think about a modified engine JUST for this purpose.
Turbocharged engine, with mild cams for lower RPM peak output (Where you want to run, or close to it), running lean -> slightly below stoichiometric value (Complete burn, slightly more air into engine than required for complete burn). This kind of engine could reach upto 30% efficiency.
Now we can calculate from energy content and alternator efficiency.
http://en.wikipedia.org/wiki/Gasoline#Energy_content_.28high_and_low_heating_value.2913kWh/kg or 9.7kWh / liter.
Let's assume we tune this engine for 30kW load, this means we need to burn 100kW worth of electricity, dismissing alternator disefficiencies. After alternator efficiency we get perhaps 27kW of electricity.
That means just more than 10liters per hour.
Please note: I chose 30kW load as load is required for something of that displacement to get to good efficiency rates, even 30kW might be far too low, but let's give it a chance for being tuned for this usage.
At the gas rates like here 95E10 going for about 1.5€ per liter = 15€ per hour and we get under 0.5€/kWh efficiency.
The remaining 70kW could be used to heat your house, or alternative methods used to collect electricity from it. But even at 100% efficiency in all parts it would still cost 0.15€ kWh which is more than double what i'm paying locally right now.
When you are moving it's not free either, but small load disappears in to the existing inefficiencies, 20RPM lower idle RPM don't matter. Even if you are going highway, and trying to get best MPG possible, it will show up there, but it's marginal.
Teal: Big Diesel has higher efficiency than tiny 4 banger. Diesel as a fuel has higher efficiency from chemical to kinetic energy.
Reason tiny 4 bangers gets so high MPG is because they are lightweight and small surface area, not exceptional engine efficiency.
For a gasoline engine 25% is pretty much the norm efficiency rating, depending upon how modern it is.
Many of the tiny 4 bangers getting huge/good MPG (80s-90s Renault Clio, 80s Peugeot 205, Subaru Justy, 80s Nissan Cherry) actually has kind of low efficiency engines in them. They are simply lightweight cars. Subary Justy actually has 3 cylinder engine in it, 1.2l carburated, very peaky output, very low torque at low rpm, even normal driving requires some 3k rpm yet even 4WD on during winter you can achieve better MPG than many big cars. Summer and no 4WD it can achieve 50MPG.
