It doesn't seem too unreasonable. Based on some quick searches, bulk cheese breaks down to about $.19 a slice, two pieces of bread is about $.10, butter is wobbly here because I don't know exactly how much they'd be using, but let's say half an ounce/1 Tbsp is about $.25? Probably not a whole lot of profit after the cart and rent for the space, but you could probably get close to breaking even if you sold enough and/or had a better bulk supplier than what I can see with 5 minutes of research.