You could use a variable to make the calculation and use toFixed when you set the #diskamountUnit element value:
var amount = $("#disk").slider("value") * 1.60;
$("#diskamountUnit").val('$' + amount.toFixed(2));
You can also do that in one step, in the val method call but IMO the first way is more readable:
$("#diskamountUnit").val('$' + ($("#disk").slider("value") * 1.60).toFixed(2));