Your fixed costs (rent, labor, etc) will need to be paid regardless if you run the ad or not. Therefore, they cannot be considered in calculating your break even. As well, unless you ad extra labor to cover the extra business generated from the ad, one can argue that you cannot consider labor cost as well. All that is left to be considered is the 25% food cost of each order directly related to the ad, which leaves you a gross profit of 75% and a break even of $893.33.
Keep in mind that you also need to consider the long term value of any new customer gained as a direct result from running the ad. I generally look to break even cash flow wise from the ad in the short term and plan on the long term value of the new customers that I retain from the ad to actually make me some money/profits.