Alright, this is a low-effort meal, so you can probably get away with one employee on a cart. Assume you have two shifts, so you can stay open 16 hours a day. Rent for a small cart can be around $600/week. Wages, assuming 7 days a week, $13/hour, are about $1456, for a base overhead of $2056/week. You have 112 hours to make that back. At a profit of $2 per sandwich, you need an average volume of just over 10 sandwiches/hour. A little close for comfort, considering you probably can't do much more than 12/hour without stressing the cook (5 minutes prep time+register), maybe 24/hour if they're fast. So, double that margin and call $9 a fair price. At that price, you break even at 5/hour, and walk away with a cozy ~$2000/week if you can hit 10/hour avg. That gets eaten by taxes a bit, but it's assuming you're mostly hands-off. If you man the stall yourself, it bumps up to a solid $2700/week profit.

$11 is just nuts. If you can get away with it, more power to you.