Take for instance this stamp:
The catalogue value is minimal, but I have never seen one of these precancels and there are 72 different ones of them!
They are out there, but they are certainly not commom. I have two, so just the other 70 to go!
"Do you have a some examples of the harder to find precancels?"
One thing to look out for is precancels on non-definitives.
You can find them on Postage Dues (nothing like as common in Belgium as they are in USA)
And here is an example on a commemorative charity issue:
Note that it has a surcharged revaluation for precancel use. SG lists and numbers all the definitives which were surcharged for precancel use - but does not list this.
For general readers who are not sure what I am talking about - these are they:
Not to be confused with these ones which were surcharged for postal purposes, and then subsequently precancelled: