I Was At A Coin Shop And

I find that unlikely but hey, someone might believe it is worth that much. It also depends on the coin. Since I don't see the photo, I don't know.

I believe the shop owner probably knows little about ancient coins. Most dealers dislike ancients because you can't grade them without an extraordinary amount of 'personal' opinion. You could have 10 PCGS AU58 ancients of the same variety and while it might take a minute to find which one is yours it should be fairly easy. I see people asking 'stupid' prices all the time especially with so-called graded pieces. In fact the dealer may have had his own inventory of slabbed ancients which he would be thrilled to sell you for 50% off ! No real collector wants to view his coins through a window ...... That's what museums are for.

Are you sure it was a PCGS slab? PCGS does not usually slab ancients. At one stage they did slab a few of the more famous types (like this "thirty pieces of silver" Tyrian shekel), when sent in by big customers in among a large batch of "normal" coins. They stopped doing this when they realized they were being neither accurate nor consistent in their grading and that their reputation could suffer, especially if at some stage in the future they did decide to initiate an ancients slabbing service.

To illustrate how abnormal this is, run a Google Image search for "pcgs slabbed ancient coin". I doubt you'll find one.

NGC slabs ancients, but don't use the Sheldon grading scale. ICG is the only company I know that routinely slabbed ancients and used Sheldon on them. ICG was notoriously unreliable, both in identification and grading. If it is an ICG slab, then without even seeing it, I have prejudged it: the coin may not be of Constantine, and would be lucky to get an EF grade.

Maybe a Constantine Siliqua or Argenteus? Those tend to get pricey.