I used Amazon.com to check the price of each card (as of October 19, 2014), and calculate what is the price you pay per kingdom card when you buy an extension.
Moreover, I used
silverspawn's rankings to calculate the price per "good" (i.e. not terrible) card, where a good card is one that is ranked 3 or more by silverspawn.
I also added a prince per "well-designed" card, as
ranked by silverspawn here. Here a well-designed one is one not ranked in Chapter I: The Bad Cards or Chapter II: The Weak Cards (And Outliers).
Expansion Base Intrigue Seaside Alchemy Prosperity Cornucopia Hinterlands Dark Ages Guilds Promo
| Price $28.22 $29.73 $28.70 $20.20 $29.22 $19.09 $27.50 $25.87 $20.34 $2.50
| Per card $1.13 $1.19 $1.10 $1.68 $1.17 $1.47 $1.06 $0.74 $1.56 $2.50
| Per good card $1.57 $1.42 $1.20 $2.20 $1.33 $1.59 $1.15 $0.78 $1.56 -
| Per well-designed card $1.28 $1.35 $1.25 $2.89 $1.27 $1.59 $1.15 $0.78 $1.56 -
|
As you can see,
Dark Ages is by far the best bang for your buck, giving nearly twice as many kingdom cards for the money as the smaller expansions (
Alchemy/
Cornucopia/
Guilds).
Although
Hinterlands cost slightly less than
Seaside/
Prosperity, you get more "stuff" out of the latter two expansions (playing mats & coins for the former, Colony & Platinum for the later).
Base &
Intrigue give a price per card similar to
Hinterlands/
Seaside/
Prosperity, however both the original sets have many duds, which inflates their price per good card. Still, you get the money/victory cards only with
Base/
Intrigue, making buying one of these sets attractive.
Guilds &
Cornucopia are among the most pricey sets - they cost about 40% more than
Hinterlands/
Seaside/
Prosperity. This is not surprising, given that
Guilds &
Cornucopia have half the cards of bigger sets yet only cost $6-7 less.
Finally,
Alchemy is the most expensive set for the cards you get. This is especially true if you don't could the "bad" cards of
Alchemy (Transmute/Herbalist), in which case you pay nearly 3 times as much per card as
Dark Ages.