If you really want to save money try using real wood. I can go to Academy and buy a bag for $10. I use the charcoal to start the firebed then go to wood. I used to go through 3 bags of charcoal to do an all day cook and now I only use about 1/4 bag of wood. That's only about $2.50 in fuel for an all day cook. The results of the meat are a lot better too using wood then charcoal.
The last time I used my webber grill I did two chickens. I used half a starter full of charcoal as a firebed and only 1 pecan and 1 hickory log for the 2 1/2hours it took to cook. It's worth looking into using real wood.