I found that the assumption that a card's power level is always directly related to its cost is a false one.
A card's cost only functions to limit its availability.
The reason King's Court costs $7 is to make it harder to get a bunch of them easily.
There is a relation to the card's power, but it's an oversimplification to say: This card is super powerful, it should cost at least $X.
Especially if you're only ever getting one copy of it.
And even if you want multiple copies, you still have to be careful with pricing if you only look at raw power.
Take Fool's Gold for example. Every FG other than the first provides more value than Gold! And just 2 FG's played provides more value than 2 Silvers. So maybe it should cost at least $3? Making it cost $3 makes it harder to get multiple copies, making it a less viable strategy and dropping its fun factor.
So in deciding on a price, you should look at it this way: If it's cheap, players can more easily get multiple copies of it. Is that a bad thing? If so, up the value. Allowing two players grab multiple Mountebanks early quickly turns a game into a slog hell. It's no big deal if a player wants two Chapels. It's not a bad thing if players get multiple Fool's Golds.
Also, there are only 3 effective price levels in Dominion.
The first level is $2-: Cards costing $2 (other than opening cards like Chapel, Pearl Diver for instance) just give you something to buy to not feel like your turn is wasted
The second level is $3/$4: Cards at this level are pretty much interchangeable. Masquerade and Ambassador could cost $4 and not much would change. It's so easy to hit $3 and $4 that you're often buying $3 cards with $4 to spend.
The third level is $5+: $5 is the level at which the "power cards" like Mountebank and Wharf are found. Anything above $5 like King's Court at $7 is in pretty much the same league, but just a bit better.
I find that the biggest gaps are between $2 and $3/$4 and between $3/$4 and $5.
So this is also something you can take into consideration when pricing a card.