Cable Cards pricing is determined by the provider, but is usually not very much. I have single stream cards from Charter in Michigan, and they are $2 each. They report they are going to send me multistream cards as soon as their infrastructure is completed to handle them.
On my single stream cards, they are one way (no PPV or On-demand) but the new ones (CableCard 2.0 spec*) are supposed to be interactive.
* (from above) There is a lot of confusion on CC 2.0. Some providers report cablecard 2.0 when they actually mean multistream cablecard (that may or may not have sending capabilities). Other providers call them interactive CableCards.
I have heard of providers charging anything from nothing up to about 6.75 for them.