Premiums over spot fluctuate for many known and unknown reasons. All it takes is a fast look at APMEX to notice that all quantities of silver dimes carry a premium of $1.99 over spot. There's no indication these are exclusively Mercuries, so why are they so pricey?
Quarters are $0.99 over spot. There's nothing awe-inspiring about a circulated Roosevelt dime. This pricing disparity isn't apparently exclusive to APMEX. It seems to have bled through even on e-Bay where the bids or fixed prices are higher for dimes on a comparison basis (overall silver equivalent) than for G Washingtons.
My LCS has no explanation for this mystery, either. Then again, I guess I still don't understand how my iPhone works, either.