Depends on condition. I picked up my 81 for $1300 on ebay, the price was a little high for the work it needed. Granted the overall condition of the bike was great. It needed the carbs gone through and cleaned, the tank had some bad patch work done to it from the previous owner, I ended up buying a replacement tank OEM from Yamaha, and it needed an exhaust system, since it had rust holes in the mufflers. The paint was good, other then the bad patchwork on the tank, the seat was perfect, it needed tires, and it ran as good as a bike could that had plugged up carbs and needed a snyc.
I spent nothing to rebuild the carbs, just time
about $300 on the OEM tank, which I figured buying one used ($80-$150), getting it acid dipped and lined($100-150), and
then painting($75-100) would have been about the same cost, and the OEM tank is perfect, no dents!
and after buying two different exhaust setups before I got a good one, or at least one that would stay on the bike and wasn't too rusty, spent about $400-500 on that
I ended up replacing the rear coil overs on it, but I did some research and upgraded them to the 1982 XJ Seca Turbo Air Ride shocks, and I will never go back to those soft unresponsive coilovers again. Best $50 I ever put into that bike.
If I were to sell it today as is, Which is close to damn near perfect, short of stripping it to the frame and re building it from there, I could get probably $1500-$2000 for it.
My dad just bought a 1981 XJ650, same as mine. Has the discontinued Jardine system that I can't find anywhere, and want really bad on it, its all flat black and the tank is really banged up, but it runs pretty good for $600 with another tank.
Theres some XJ specific forums out there that are usually good help, and can get you a good idea on prices as well.
http://www.xjbikes.com I think is the one that I floated around looking up info on for a while.
Overall they are great bikes, they share a lot of similarities with there big brothers, the XS models. The inline 4 engine is of the same family, the carbs are similar other then size, hell my XS1100F even has the same front tire, and if I had a XS1100SF, the special model, it would even have the same rear tire.
The only real downfall I have with mine, being a Maxim it only has a single front brake, which is very under powered for the bike, even with just me riding it I cannot stop the bike in a timely fashion with out engine braking the whole way. I think there is something actually a miss with mine, and now that winter is here I will look at it. I have wanted to switch mine over to the XJ Seca forks and dual brake setup, and plus if I find an XJ Seca RJ setup it has air assist front forks as well!!!
Hope that helps some. I'm no expert by any means, but I have owned mine and worked on getting it up to par for the last few years, if you have any questions I can try and help.
This is mine when I got it.
1979 Yamaha XS1100F standard, currently naked, Jardine 4into2 system w/equal length tube header
1981 Yamaha XJ650 Maxim