What is it worth then?

On average you pay $10 for a Compact or 68030 machine, but normally you'd get it for free, as these models are not worth much to a company. Most private owners refuse to sell their machines to a market price and should be avoided. The size of the Hard Drive, installed options and the amount of installed RAM might influence the price.
A NuBus PowerMac or a 68040 machine might go for around $20 - 50. PCI based PowerMacs with a 601 processor for $50 - 80. PowerMacs with 604 processor might go for $75 - 150.
If you add a screen, you could add another $10 on top of the price. All of the above prices are including mouse and keyboard and should be in a working condition. If you want to buy or sell a broken vintage machine it is not worth anything. But then again you might be able to buy or sell a broken machine via eBay. Give it a try.

If you got or want a collectors item, now that is different. Most Apple machines would not be considered collector's items, but there are a few worth paying extra for. Notably are of course: the Apple I, Apple II (original), Mac 128k, Lisa, MacTV, Twentieth Century Mac, Pippin, Set Top Boxes, eMate and some Workgroup Servers. Perhaps we will also see the Newton series and some developer machines worth a whole lot more.

Most western countries have a very short turnaround time for computers and in Europe some countries even exchange their total computer park every 3 years! For tax purposes computers are usually considered worthless in that time and can then be terminated without any tax penalties. 3 years and your computer is worthless. This is good for those of us who are collecting, but if you think you are going to become rich from your Apple collection . Think again - Think Different!™.
Remember that the computer you once bought for a specific purpose, might still do the trick and if so, is it really worthless? Certainly not. You can still operate it the same way and in many cases faster than most over loaded computers of today. But then you might not get color, internet and comfort.

So what ever you do:
Don't pay too much!
It just isn't worth it..