I don't think you can think linearly about card costs.
I mean, getting a Colony is much much harder than getting a Province ($3 difference), but getting a Silver isn't much harder than getting a Copper.
So while adding a small bonus to a low costing card can bump it from $2 to $5, adding the same bonus to a $7 card may only bump it to $8. And not all bonuses are created equally, the card that it's applied to matters a lot.
Imagine that there would be a card called Palace or something that's basically a King's Court with an extra Action, it doesn't need to cost a lot more because King's Courts themselves are often used in engine-y games which already give you plenty of actions. So that extra Action doesn't give you much extra benefit over a regular King's Court. On the other hand, if we make it a +1 Card King's Court, its usefulness does increase substantially as the gist of a KC-engine is to KC into other KCs. If you Palace one Palace you already end up with 4 extra cards which makes it a lot more easy to find other Palaces and action cards.
And as mentioned going from terminal to non-terminal is huge, because cantrips and extra Actions are the heart and soul of an engine. So it really depends on the card what small bonus would give a huge boost to a card's ability and cost. Engine cards need to find other engine cards, that's why giving them +1 Card is huge for them. They usually provide coins in some way so adding +$1 isn't all that extra powerful. On the other hand, adding +1 Action to terminals is huge because it makes them much more viable in engines.
I tend not to look at costs as absolute power indications, more as a way to control the availability of such cards. Chapel is $2 so it's available for every player at the start, that's probably a good thing. Goons is $6 so it's very hard to get on your opening turns or even the second shuffle. The power $5s are bad if your opponent starts 5/2, but if you both start 4/3 you have to invest during your first turns to be able to get $5 reliably on your first reshuffle. Why is Forge $7? Well, maybe because in games in which Forge is actually useful (big hands), you're able to draw $7 easily anyway. So by making it $7 it actually tries to make sure it's only used in decks in which it's useful. I've never thought about it this way, but it seems pretty clever. Expand is a $3 jump over Remodel because of its much increased potential. As there are likely more $5 cards in your deck than $6s, being able to turn them into Provinces instead of just Golds is a pretty big upswing. For example if you have 3 Golds and 6 $5s you're looking at a 200% increase in "cards that can be converted into Provinces".