Vintage Bernina's retain much more value than many other older machines. What they sell for depends somewhat on where you live and how many quilters/sewists there are in you area. Usually there are more machines on the East Coast, fewer in the Midwest, and least on the West Coat so supply and demand can vary considerably from one part of the country to another. Here in WI, someone recently paid $500 for a Bernina 830. Price also depends on condition and on what comes with the machine. A machine with a case in good condition will sell for more, as cases are expensive. A missing power cord or pedal will decrease price by $100 or more simply because replacement costs are high for these.
It's a good idea to check eBay for "sold" prices on the specific models you have. To do this you first need to create an eBay account (it's easy and free). Once you are logged in, search for the Bernina model and then check the "completed listings" box that is one of the options running along the left side of the screen. When you look at these, be aware that only the prices in *green* indicate the item actually sold; black prices are listing price for items that did not sell. Here is a link to a search I just did on the Bernina 830:
http://www.ebay.com/sch/i.html?_odkw...ecord&_sacat=0
You will see that there is a huge difference in selling price between a "machine only" listing and one with original red case and accessories. This kind of search can give you a reasonable ballpark asking price for your machines. A good place to sell is a local quilt guild (check to make sure you can leave a pile of flyers on the meeting admission's table, or post in the guild newsletter).