Can't go solely on mintage. Seated dimes
have many low mintage dates that are not considered keys, because there is not enough collector interest. So no buying pressure.
I think you take a series and look at a combination of mintage and availability along with collector interest and these all are important factors. There are also conditional rarities- common in low grades, but key or semi-key in higher grades- many Buffalo nickels
fit this, some Morgan dollars
also fit this.
I think in some cases, key dates are overpriced due to apparent scarcity that really doesn't exist, but the seller just doesn't want to sell it below a certain price. So many eBay auctions on keys/semi-keys anymore are buy-it-nows for inflated prices.
Here are some that I would consider key dates (not all-inclusive):
Cent: 77 Indian, 09S Indian, 09S VDB, 14D, 31S
Nickels: 13S Type 2 Buff, 21S, 50D Jeff (thought mintage is not super scarce- this may be an example of an over-hyped key)
Dimes: 16D Mercury
Quarters: 16 standing liberty; 32 D, 32S Washington
Halves: 21P 21D Walking Liberty halves
Dollars: 89CC Morgan, 93S Morgan, 94P Morgan, 28 Peace
Here are some that I would consider semi-key dates (not all-inclusive):
Cents: 08S Indian, 09S Lincoln (might be considered a key), 24D
Nickel: 26S Buff (lowest mintage, but apparently saved enough), 31S Buff (2nd lowest mintage, also saved)
Dime: 21P & 21D Mercury, 96-W Roosevelt
Quarter: 27-D and 27-S Standing Liberty
Half: 38-D Walker; 70D JFK
Dollar: Most of the CC Morgans, 21 Peace (could be considered key by many)