First, find the serial number on the machine and look it up on this website:
http://www.singerco.com/support/machine-serial-numbers
That will tell you what year it was made. Then do a google search putting in the terms "singer sewing machine" and "year[the number of whatever year you found above]".
The postings you find will include Amazon, eBay, etc sales and you can see what kinds of prices people are expecting to get for this model machine. DO NOT GIVE THIS MACHINE AWAY FOR A SONG! It is valuable even if it is not working right now. It can be fixed. I love it, but live too far away to come to your yard sale ... {sigh}.