Harold,
Obviously you wouldn't want to put more money into a 645 than it's worth, but the question is -- how much is it worth?
For comparison, the absolute cheapest new 3-manual AGO organ you could buy is surely $30,000 -- and that is Cheap with a capital C -- made for the low-end market. But, even that cheapest new organ would be fully digital and would have MIDI and perhaps a sequencer or other features that were not available back in the 80's when the 645 was on the market. However, the 645 was, at the time, pretty much a premium deluxe model, with a very nice console, excellent keyboards, a sturdy pedalboard, and a nice audio system -- so comparison to a super-cheap new organ might not be quite fair.
Maybe a better comparison would be with a comparable used organ. I occasionally sell a used organ to a church when one falls into my hands. Not too long ago I sold a nice used Rodgers from the 80's (though it was a 2-manual, not 3) to a church for $5500 -- and that price included delivery, a set of new speakers, complete installation and setup, voicing and tuning on site, and a year's warranty. Of course I'm not a dealer, and a dealer might have gotten more for that organ with the same set of extras, so you might say is was worth $7500.
I'd guess a dealer might try to get $7500 for a very clean 645, delivered and installed, with some kind of brief limited warranty. So you won't be going terribly wrong if you sink a few thousand dollars into it, but obviously you're not getting the extras and services that you'd get if you bought it from a dealer. You'll be on your own as far as moving it, setting it up, and making any repairs that may be needed, and those items could mount up quickly.
OTOH -- I see a lot of perfectly decent organs not over 20 or 25 years old that are free for the taking. And I tend to tell churches that are trying to get rid of one that they'll probably wind up giving it away just to get it removed. So, you don't necessarily have to pay much for the organ itself. Where you wind up spending your money is in getting it moved and then fixing it up.
Hope that helps a little. Maybe someone else will offer more information.
John
Rodgers 890 at church.
Baldwin D422 at home.
Scads of old organs in the shop! H E L P !!!