Can Someone Explain Premiums To Me?

Did they have many coins? It's possible they just pulled a number out of the air. Or set it high expecting you to haggle - I mean, who pays asking price at an antique mall?
But yes, you would have overpaid. I see a BIN listing for this coin on ebay for $16 with free shipping.

I think Hondo gave a good answer for the situation you descibed.

Which leads us to one possible explanation (definition) of premium: the difference between what a seller hopes they can get compared to other assessments of the fair market value of the object.

There is another type of numismatic premium which is the slabbing premium. That one can be defined as the added amount a buyer is willing to pay to have a recognized expert tell them a coin is authentic and what its grade is. Or, from the seller's side, it is the added amount they add to selling price to cover their slabbing costs. Those two premiums don't necessarily match (for example, when an inexperienced seller has paid to slab a coin for which the market pays little or no premium).

A third type of premium is the one between the sticker price of a coin at a brick-and-mortar store and the hammer price of the same coin at an auction. The dealer can charge 25% more for the same coin because the final cost to the buyer of the auction item will typically have a 20% commisssion and insurance/shipping charges added.