Alright, my thoughts on what the cost of cards should mean about the card:
$1 - Slightly weaker than $2 cards, Copper upgrades into it, generally indicating that it works better without Copper. Easily acquirable to every player. (There is only 1 Official $1 cost card, so this may change)
$2 - Available to every player for a certain reason (Heavy early trashing, powerful in multiples, etc.), or VERY spammable
$3 - (Hopefully) Never strictly better than silver, doesn't cause problems opening with 2 copies.
$4 - Similar in power to $3 cost, but could be too powerful opening with 2 copies.
$5 - Power cards, stuff that you should not be able to grab itself and a Silver as an opening. Note: a lot of the cursers are in this price point.
$6 - Very powerful, multiple copies may be devastating (Goons, Grand Market), or very good in engines, so much so that they should NEVER be able to be opened with.
$7 - Pretty much the same as $6, just a little harder to get earlier.
Anything outside this range should be heavily tested to make sure it's good at such a cost.