you're looking for the weakest cards (defined as the cards worst at fulfilling their roles)
Theory, I take it you agree with my critique then? You are actually evaluating the relative value of cards with similar functions in a list that purports to be evaluating the relative value of cards with identical cost.
So I think of "The Five Worst $X Cards" as looking for the cards that cost X and that are F or D in its role.
Obviously you can think whatever you want, but your thinking here is ... flawed. A crappy trasher can be stronger than an equally-priced and moderately effective deck inspector if trashing is much more valuable than deck inspection. Let's put it this way: each card's value is a product of the value of the role it plays (draw, trashing, cycling, sifting, actions, etc.) and how effectively it fills that role (fishing village provides +actions better than other cards that serve that same purpose). If you want to rank the $2 cards, you need to calculate this for each card and then rank them. What you are describing is actually a two-step process: rank the cards by function, then group them by price while trying to keep each card near the same rank it had in the by-function list. That is just nonsense.
If you decide that Cellar has some rank relative to the other $2 cards based on how often you want to sift/cycle and how well Cellar does that, and then Warehouse comes along and serves the same purpose but does it better, how could that possibly affect the ranking of Cellar vis-a-vis the other $2 cards? Compare: on average, Witch is a better card than Counting House, using whatever metric we're using for these comparisons. Now I invent a card called Cheap-Witch which does exactly the same thing but only costs $4. Given that Cheap-Witch is strictly superior to Witch, you will never buy a Witch, right? So now Counting House is better than Witch, right? Of course not.
The only way I can think to save your argument here is to say what we're really talking about is how frequently you will want to buy a card. Cellar is better than Moat, but since Warehouse fills the same role as Cellar and is better than Cellar, you'll actually buy Moat more often than Cellar. But that's pretty weak reasoning given how rarely Cellar, Warehouse and Moat will show up in the same kingdom.