Well, nobody else seems to be picking up on this one so I'll give it a go. For starters, I would strongly advise against obtaining anappraisal from an antiques dealer. Many people make the mistake of consulting antiques dealers on these vintage organs simply because they're old - but you really need a dedicated organ dealer,or perhaps a professional organ technician,to get a firm idea of the market value. Antiques dealers seem to try to evaluate them on the basis of age and cabinet condition alone (in other words, as if the organ were simply an antique furniture piece), and they almost invariably end up telling the poor owner that their 1972 Thomas Californian is worth $5000 - $6000! (which is absurd; such organs are typically given away or sent to the dump, because nobody wants them any more.) And calling a general music store for an appraisal would probably be an exercise in frustration - you might well get someone who has never seen an actual Hammond organ, much less know anything about the comparative value.
But you're fortunate in that you've got one of the few old organs that's actually worth anything on the market. The A100 is identical to the famous Hammond B3 internally -- same tone generation, controls, features etc. -- except that it's built into a different cabinet with console audio, including reverb. Now, from what I've seen prices vary by region, but your A100 should be worth a minimum of $800 - you'd probably get a very quick sale if you price it at that amount. However, again depending on your area, you might get a lot more - perhaps double that amount, or just possibly even more, if it's a particularly well-preserved specimen. If you have the finished back piece, that adds value; also, you mention it was bought in c. 1960. This means it may be one of the early models with "necklace" style reverb springs - these early models also tend to go for a little more. (If you need to see pics of the finished back or the necklace reverb, post back.) But of course,there is an upper limit even to the really nice ones - for instance you may see ads for A100s offered at $3500 - $4500... but these never sell at those asking prices.On the other hand, adealership might be able to get that kind of money, but they've usually gone through and rebuilt amplifiers, etc. - so they can offer a warranty and some kind of service plan for the price.Butthe range for a private sale would be something like $800 - $1700 (US) from what I've seen; or maybe up to $1900 for a tip-top specimen with necklace reverb and finished back panel. Again, this is just from what I've seen personally.
It also helps a lot if you can honestly state that the organ is in 100% working condition. So if it hasn't been oiled in awhile, oil it (again post back if you need help); and sit down and run through the keys, functions etc. Most tonewheel Hammonds run fine with very little attention; and even those that need work can often be brought back up to snuff with minimal effort. --Oh, and I almost forgot! - you didn't mention a Leslie, but if there's a Leslie speaker with it that's a whole different ballgame. Vintage Leslie models 122 can easily go for $1000-$1500 just by themselves; slightly less for a nice 147 or 251.
Anyway hope this helps. Good luck with it-