There is no agreed value to a Singer sewing machine. Much will depend on factors such as condition, style and what collectors are willing to pay, based on what they consider valuable.
There are many collectors seeking antique sewing machines. A machine's age is recognized by its serial number. If it begins with the digits 500 or less, it is considered a rare piece. This will increase its overall value. Each collector has varying personal desires that will affect how much he is willing to pay. Some may look for age, while others are more concerned with style. Some collectors value machines made by obscure makers higher than those from more mainstream manufacturers.