Opinions On Pricing Of Mid-Grade Coins?

It depends on the price range of coins. Unless extremely rare, key year/more expensive coins do not do well in EF/AU grades (will generally sell well below trend). Collectors with means to afford those price ranges are generally looking for better coins (and can bankroll better grades). Collectors who don't have the means for expensive coins frequently are unwilling to buy them or afford them. I personally don't like mid grade coins. Especially if readily available in higher grades.

There's a couple of older threads on this. If I remember correctly, the stats showed that using 1/3 instead of 1/2 was a better approximation in most cases. IE if VF20 is $20 and EF40 is $50 then VF30 would be $30 instead of $35.

In the last couple of weeks I've been trying to sell a couple of AU-58 items. On the basis that an AU-58 is about two-thirds of the way between an AU-50 and a MS-60, here are my results:

Canada 5 cents 1902: Trends $34; realized $21
Canada 10 cents 1912: Trends $210; realized $76

Fortunately, I paid nowhere near Trends to buy them.

Generally speaking, harder to sell than a MS-60 of the same vintage.