I've been looking at the cards from Dominion: Alchemy and want to use some of them in my own games through proxying of the blank cards. However, I don't feel like delving into the whole potion thing, especially since it would be annoying to have to proxy at least 4 different cards (the potion and 3 kingdom cards to get the recommended amount of 3-5). So I figured I'd just give them costs in $ only and not potions. Another application of this might be playing with the alchemy cards if for some reason you didn't want to use potion as well.
However, I'm not exactly sure what costs to use. A potion is roughly equivalent to a silver, but that seems like an oversimplification. One of the interesting things about alchemy is that you have to buy the potions in order to access these cards and balance them with your other currency. This scarcity makes me lean more toward counting a potion to add +3$ to the cost.
Does this seem about right? If not, could someone perhaps provide a case by case basis for the 9 cards this would pertain to (since Alchemist in a potion-less game would be rather odd I'm not planning on including it)?