Seems like the Dolby processors are difficult to find these days while processors from Smart and Ultra-Stereo are more common.
That probably answers your question as well as anything. The Dolby gear is good stuff, and probably doesn't get changed out unless a theatre upgrades or closes. Even then, that equipment normally finds itself on a shelf or upgrading another of that owner's screens.
I have CP65 processors here, and am using both Dolby and DTS digital units with them. Can't see any reason to change, unless you want new... then CP650s and Ultra Stereo stuff comes up in regular conversation.
Not that Smart and Ultra Stereo don't have some decent gear out there... but I'd be careful about using some of their older stuff. As far as used equipment goes, most of what I hear (and have experienced) is that a CP65 will put out as good an analog signal as you're going to get. You want that if for some reason your digital switches out on you.
I'd get a list of theatre supply companies & see what range of prices you might find.
If you're on a budget, Smart has a new processor out. I've heard a couple of good reports about it, but not enough to know one way or the other. Maybe something to check out, though.
Here's the link:
http://www.smartdev.com/cin_prod.htm