The amount a concession trailer should expect to gross at a festival and county fair per day is majorly determined by what you are selling, who you are selling to and the area you are located. The cost of starting a concession stand usually breaks down to 25% for food, 25% for labor, 5% for power, 25% for miscellaneous costs, 5% for insurance and maybe 15% pre-tax profit. Visit http://www.ehow.com/about_5286564_foods-sell-concession-stands.html for additional details.
My suggestion is to go observe a concession selling what you intend to sell. Count customers over a period of time and create a model of the gross income during peak period. Try to figure out how many hours are peak. Otherwise ask the vendor for information. They might be helpful.
It really depends on what state you set up in and what you are selling. Our concession stands grossed approximately $1000.00 each and we had two stands. You can't really go wrong with hot dogs and pop, but these days it might be good to have sparkling juice and all beef natural hot dogs on the menu for the health minded. Good luck! : D