... if Fortune costs $8, doesn't it stand to reason that when you buy Fortune, your deck can somewhat consistently produce at least $8? It basically is, at a minimum, an extra Province a turn for a deck drawing engine.
Sometimes, but often with a strong, expensive card on the board, you use stuff like Death Cart to spike whatever you need to buy it. Don't forget the
. As is often the case with expensive cards, you need to decide if you want them over Province/Colony. Basically, the more expensive the card, the smaller the window of opportunity for it's use.
For example, if you can consistently hit
with 1 Buy in a two-player game (not a rare situation), then buying Fortune slows your deck down by 2 turns. Assuming you are both getting a province a turn (as often happens in single-buy kingdoms), you need to CAPITALize on Fortune twice in order to offset the cost of buying it in the first place.
If you can hit over
often far before your opponent, you will probably win already.