It would depend on a number of things. Do you know any traffic history of the event (how many people)? How many vendors? Any direct competition vendors? What will your total costs be (operating, consumables, cost of goods sold, labor, etc)?
I ran a quick calculation for a basic vending stand at 10 hrs/day (using some averaged costs based on similar gigs we have done in the past to net out $700 profit for the day) and I came up with a required gross sales of ~$2K/Day. If your average dollar per sale is $8 then you would need to average ~25 sales per hour to meet your desired goal. Of course this is just an educated guess since there are many unknown variables in this proposal. But it might give you some idea.
To give you something to compare to we do a one day festival event every year that pulls in about 3,000 people for the day (10am to 4 pm). There are multiple food vendors and usually 2-3 other BBQ vendors. Approximately 80% (240 plates) of our sales occur from 11 am to 1 pm. We sell a total of ~300 plates for the day @ an avg of $7/plate for a gross of ~$2,100. We are able to keep our costs low so we usually net about $1,500 for the day.