Thanks for contacting ManageMyLife.com. I know how important it is to have information on your grill. Unfortunately, there is no way to determine this. Although it may only be used 10 times a year, there's no way to determine the price you'll pay for propane or charcol, how long the grill will be in use or the condition of the grill and valves associated with it. There's also a lot of variables, including the flow rate, the BTU output of the burners, and how high you run the burners. Generally, you would need to do test runs to determine cost effictiveness. Ususally, propane or gas comes out to be less expensive though. Sorry for there's not more information on this.
Please feel free to let me know if you need further assistance.
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