For a fully functioning Bally Computer System model, which is pretty common, I would say ask at least $100 and settle for no less than $75.00. Just do yourself a favor and really play the system for several hours on any type of level surface other than a carpet. The systems are prone to failure, but my experience is if you can play it for several hours without any glitches or it crapping out completely, you've got yourself a keeper. Less of a headache in the long run with someone calling you back to say the system just died on them. (Just make sure they know not to play with it on the carpet).
I see these systems going up in value in the near future, especially with the recent homebrew release of War and the upcoming release of Crazy Climber. (A little birdie also told me that bigger or should I say larger things are coming on the horizon).
For a White Montgomery Ward version of the Bally Professional Arcade (which I assume yours is) I'd say you got a great deal. For a pristine console that is still white and not off-white or yellowing, easily $150.00 to $200.00. I rank that model as the third rarest version. Nice score.
There really is no "matching" of boxes, manuals, etc. when it comes to this system, except for perhaps the earliest system which by the way was the Home Library Computer, not the Bally Computer System. The systems history from Bally, then Astrovision and finally Astrocade, boxes, manuals, controllers and consoles where all kind of "thrown" together like a box of chocolates. So basically you never know what you are gonna get.
Sites like Bally Alley and THE Bally/Astrocade FAQ can fill in a lot of information on this:
http://www.ballyalley.com/
http://alteeve.com/~lance/Ballyfaq.html
There were four different "names" for this system:
1) Home Library Computer
2) Bally Professional Arcade
3) Bally Computer System
4) Astrocade
The Home Library Computer model was by mail order only, so I suspect whatever those systems came with would have matched that era console.
The Bally Professional Arcade, the second release, came in both a woodgrain finish as well as the white finish (the white being the rarer of the two). Then came the Bally Computer System and finally the Astrocade brand. I rank the Home Library Computer model as rarest, followed by the Astrocade and then the white Montgomery Ward Bally Professional Arcade version.
They are also the only ones I keep on display: