I'd say in current condition (need servicing, handlebar tape, a seat, and new tires and tubes, which will cost you $200 or more, depending what you buy and who does the work), a realistic number would be $150.

In top condition with a tuneup and new tires, ready to ride (what does the other side look like?), it isn't worth more than about $350 if you are willing to wait for the right Buyer.

Can you do the work yourself, how much are you willing to invest to get more out at time of sale, and how long are you willing to wait for the right Buyer to come along? Depending on your answers, you might be better it off selling the way it is.