Hi Mike
Bentleyman has given a fair estimate of the 9500 related stuff, so I'll have a bash with the remainder:
1900
Always going to be difficult to find in fully-functioning condition. I suppose a good lot would set you back somewhere in the region of £200ish on a reasonable day on eBay. Depends to a great extent who is bidding and how much they want the system. If you were in Europe I would suggest looking for good cosmetic units that are in need of electrical repair, as this is the route I'd go down.
4500
These can vary. The Beogram is the most expensive item, and if it has a good MMC in it then it will attract more money. You have a better chance of finding good working units on eBay than with the 1900 or 8000, but watch for the glass covering the controls if you want to wall-mount, as the adhesive seems to de-nature over time and the panels can fall off. I've seen good Beomaster 4500s go for £150 on Ebay, and the Beogram CD 4500 seems to be in the same price bracket. Beocord 4500s are worth next to nothing, and I sold one a couple of years ago for £40. The Beogram 4500 is harder to price as people do buy them to pair with the Beocenter 9500 on occasion. Maybe £250-300 for a good one with a good MMC4.
8000
I have one of these systems and I have to say that it's the biggest sausage on the block. Forget trying to find a full system that works, as there will inevitably be something wrong with at least one of the units. Problem comes from trying to get these repaired in the US, as I only know two Europeans who I would normally ask to have a look at my stuff. Everything in this system is big and heavy, and will cost $$$$ to ship abroad for repairs. To give you an idea what I'd want for my system if I broke it up (which I never will), then the Beomaster must be worth £300, the Beogram £300, the Beocord (depressingly, given how good it is) about £60, and the CDX about £120. You'll be lucky to find some good MS150.2s, and good pairs are worth at least £150-200. I just got some Beovox Pentas (£500) instead, as they are easier to find, and easier on the eye.
One final thought is to consider the size of the room where you will be using these systems. A big room will swallow up the sound from a 1900 + CX50s just as sure as an 8000 system would probably be overkill in a smaller room. I have a Beomaster 2000 + CX50s in my office where I don't want loud music and to sit far away from the sytem. My 8000 setup is in a much larger room where I'm about 15' away from the Pentas.
Of course, you could just get everything you want (as I ended up doing over time)!