While not having to pay people is a plus, you still have to price based on your actual costs and not just by what others are charging. You have to know your numbers before you can decide whether or not what you're charging will actually bring in any money.
Use the 3 times food cost rule for starters. Add up all of the meat, buns, sides, etc. you have to buy and then multiply that by three. Then divide that by the number of expected plates you expect to sell. What's that number? Is it higher or lower than 7 bucks? That calculation alone will tell you how close your numbers are.
If you come up with anything less than $7 per plate, then there's enough meat on the bones so to speak that you'll make some money for the charity. But if you come in higher than that, then you're pushing it because that 3x markup also accounts for all the fuel, travel, possibly things like place settings, and all other incidental costs.
The bottom line is you have to know what you are going to spend before you can ever put a price on a plate. I mean think about it, what if you go out and feed 200 people, they all make the recommended donation, but in the end you and all of your volunteers end up devoting 10 hours of their weekend just so you raise a net of 200 bucks? Sure, it made money, but that might be just a dollar a head. That's where you can figure out that charging/suggesting a buck or two more per head can make a world of difference.
So, start by tallying up what it's really going to cost to put on the event and then think about prices instead of just estimating that because you don't have labor and because it's lower than others are charging that it will all work out in the end.