I did a search of completed listings on eBay. You can buy a new one from Target for $150 with free shipping:
http://www.ebay.com/itm/Singer-6180-...item4620241b58
http://www.target.com/p/singer-6180-...sewing+machine
Here's one that sold on eBay for $69 ($53 plus $15 shipping:
http://www.ebay.com/itm/Singer-6180-...item33a81d0618
I found these completed listings for the 6199 on eBay. Note that only green prices indicate actual sales. Black prices mean they were offered for that amount but nobody purchased:
http://www.ebay.com/sch/i.html?_odkw...+6199&_sacat=0
The low-end Singers have a poor reputation, so I wouldn't invest in a used one even if it were half the new price. Number of stitches doesn't mean much if you can't rely on the machine maintaining tension, etc. I would save up and buy a new Brother from Walmart. Also a low-end machine, but probably more reliable than the Singers.
Edit: Here is the 6180 on Amazon with reviews:
http://www.amazon.com/Serviced-6180-...dp/B005QB4BYY/
And here is the 6199 on Amazon with reviews:
http://www.amazon.com/6199-Brillianc...dp/B0058O5YWC/