How much would the cards from Dominion: Alchemy cost if you wanted to use them while not playing with potions?
One of the defining characteristics of Potions in Alchemy is that they force a reshuffle-delay.  That is, you have to first invest in the potion, then reshuffle and draw it before you're able to spend it on a card. 

This is never going to be true of a cost that is just boosted to cost more coins, unfortunately.

I don't have any particular evidence to back this up but +$3 seems high, at least for some cards. Possession might warrant a $9 cost, I'm not sure, but some of the other weaker cards really don't feel like they'd work at +$3 to me.  I'll try to come up with my per-card opinions at some point.

Bummer about Alchemist, too - it's one of my favorite cards from Alchemy. Then again, I'm a fan of big honkin' action chains.