Dollar slices aren't just a slice of pizza for a dollar, they're a different business model focusing on just sheer volume. Joe's has at least 3 employees (cashier, guy taking your order and reheating the slice, and guy making the pizzas), plus prep people in the back, and 4+ different pies, and 20+ seats. The dollar place I go to has 2 guys, they don't reheat it, any toppings are just added onto your slice not actually cooked into the pie, no seats. I'm ignoring differences in quality/quantity of ingredients because I've had some great dollar slices, but also some abysmal ones.

That said, a lot of dollar slices are now $1.50.