From my experience, if the cost of a resource goes under 1000, that means that there is one somewhere in the galaxy. I believe the market gets half the amount produced anywhere in the galaxy. 1000 means (I think) that one was upgraded to produce in the first place, and that first one at 1000 is "free". The next price down is 850 (I think) which means that 2 have been produced somewhere.
I might be wrong about some of this, but my experience seems to back it up.
(edit) Also, if there are none at all in the galaxy, or none have been upgraded to produce, they won't even show up in the market. I've seen this, when looking for a specific resource. It won't show up for a while, and then show at 1000. Which means that somewhere, there is one. It is just a matter of finding it.