(Incidence of CN2 cards in TC per pack)* (Frequency/Total) = odds of opening a particular card in a pack
0.33 * (1/72) = 0.46% or 1/218 packs for cards with Frequency of 1 like Leovold and Sanctum Prelate.
As for final cost, that depends on demand.
$200 Sanctum Prelates
If you were wondering how much people wanted to test with Leovold, $65 is your proof. What a joke. It's now more valuable than any Power card not named Black Lotus and the 4th most expensive card on MTGO. Will it stay at this price, no, but it's not going to dramatically drop overnight because it's so hard to pull one of these.
Right now prices are rising. Leovold over 70tix. I would snapsell if I opened one. After a while these cards will all be under 20tix is my assumption. More and more treasure chests are injected to the market every day and once the market is saturated with Leovolds the price will drop significately.
So I cast show and tell, put griselbrand into play, opponent slaps down leovold. I'm a little annoyed and worried but glad to get a chance to test against it.
Twice I activated griselbrand on my opponent's turn to draw ONE card for seven life
It still helped me win. The worst part was when I had to use forbidden orchard to make Mana and my opponent got a free card from it!