It does work - as limited as those lists work anyway.
The question was how good the cards are - relative to their cost. So if we ask this in a different way: Given this card in a kingdom - how often will it affect the game taking into account that there might be other cards at the same price level. Therefore you have to know how good each card compares to cards to the same price level.
The percentage thing was a method to compare to last time, yes. Especially it was needed to let people vote without having to vote for Dark Ages, that was the main reason. But this is also a good approximation for the question asked. Forager for example is sure a good card, but the competion is quite high for cards costing $3 with Masquerade, Ambassador, Fishing Village, Swindler, Warehouse and such. And Hermit and Market Square are good cards too. So Forager is a good card which you will buy quite often, but it's still none of the best 5 DA cards, I think there is no doubt about it. Then it should be at a power level of Swindler and Warehouse.