It really depends on where you live. Up here in the upper Midwest, the top price for a Singer in excellent condition (with case and accessories) is probably around $300. If it is in mint condition, more, but most machines that were actually used are not in mint condition.
I'm not sure, but my feeling is that featherweights are less expensive on the East Coast (because there are more of them in that area, which was more densely populated in the day) and more expensive as you go west and especially as you get into less densely populated areas.
Condition affects price enormously. Look especially at the condition of the gold leaf, whether there are scratch marks or adhesive marks on the bed of the machine and elsewhere, condition of the case (if there is one). Oil cans add some value to the accessories. A blue "century" medallion on the machine can add some value, as does a fancy face plate (covering the left side of the head). Fancy face plates are supposed to mean an earlier machine than a striated face plate (straight lines up and down the plate). However, over the years many people have swapped out parts so that it is quite possible to buy a later machine with earlier curlicue face plate and/or century medallion.
For a working machine only, sans case and accessories, price is around $150-200 where I am.