In general terms, start with something like NADA or Kelly Blue Book to get a starting point if the unit is listed in either. Then, just keep poking around the internet for comparable units and how much they are asking. At least, that's what I did when I made an offer on mine. I started with the lowest figure I'd seen out there and then negotiated with the seller until we came to an agreeable figure. We were happy, they were happy and now 3 years later we are still happy even with required upgrades and renovations.
Good Luck!!