Does a card costing $4+P look like the Potion >$0 so you do not gain a copy of it because it then costs >$4? also is there a difference for cards that cost $4+P vs $2+P or $3+P? Such that $0<potion<$1 so that any card below $4+P would gain a copy or is it such that the potion is a foreign currency that the talisman cannot duplicate and then would never allow for gaining a copy of a potion required card?
If a card costs $x+P, then it costs more than any card that costs between $0 and $x, and it costs more than any card that costs between P and $(x-1)+P. It costs less than any card that costs at least $(x+1) and a potion, or at least $x and at least 2 potions. It bears an undefined relationship with any card that costs at least $(x+1) but no potions.
So, for example, if you buy a Border Village ("When you gain this, gain a cheaper card.") you can't get anything that includes a potion cost because it doesn't cost less (i.e. isn't cheaper). But if you have a Haggler in play ("When you buy a card, gain a cheaper card.") and you buy a Possession, you can gain a Border Village (since $6 < $6+P) or a Scrying Pool ($2+P < $6+P). If you Expand a Possession, you gain a card costing up to $9+P, which means anything in the Supply except a Colony.
So yes, P > $0 when looking at costs, but there isn't a "value" of a Potion in terms of coins.