I hope too...
In the said article they claim about 80% savings which means about £6 an hour just for the fuel.
Also it seems that the engines are quieter and there is only the throtle control to play with. No carb icing (well no carb.)
I guess that you got your figure considering the other costs as well - eg. the cost of the modification which involves a new engine, propeller and various other bits!
Nevertheless I still think it's a good idea.