If the wood of the cabinet is in very, very good condition (no water marks, no splits, no lifting veneer) and the machine is near pristine - then maybe around $200 - $300, but it really does need to be in top condition. Most people think "good condition for it's age" which means poor condition.
Otherwise it's between $25 and $300.
Now, if you've got a few Fashion Aid accessories tucked in with it, they can easily be worth more than the machine, but these are not the basic set of attachments, but the special purchase ones.