Mexican Coins...tourist Trap? Or Rude Buyer?

Hello all,

I just came back from cancun (Tourist Trap) and was in a flea market and saw some coins in a jewler case there, figured I may pick some cool coins for cheap (around spot) as souvenirs. I am not familiar with mexican coins but they appeared to be legit 30g 5 pesos coin 1948 was what I was intersted in.

Correct me if I am wrong, the weight was correct 30g...
I believed it to be 90% Silver..not 80% when I offered my price, he told me it was "PURE"...which I knew was not true.

His starting point was ~2300 Pesos If I remember right (like 200$ US!)

I told him that that was way to much and it was a common coin (I think it is ..correct me if I am wrong)

I offered him 15$us... about spot as a starting point but I would of not went more than 20$. He then asked me to leave the store after insulting his coin....?

Was I rude and this is a rare coin and I am oblivious...or was he waiting to rip off an uspecting tourist who knew nothing about coins and wanted to buy a old "PURE" silver mexican coin.

I though spot was a fair offer (similar to a circulated Peace dollar)