Instead of putting a cost to each type of player, it might be easier to do it in terms of turns.
Buying a lv 10 - skip Turn 1
Buying a lv 20 - skip Turn 1/2
Buying a lv 45 - skip Turn 1-4?
There is too much importance in the beginning, especially in terms of trashing, that the player can have more of an impact at the start.
Another way to look at it is if the player is playing poorly and buys a lv 45 after, the lv 45 won't be able to recover with what is in the deck. OR the deck is actually pretty decent, in which buying a lv 45 would probably be not as useful as the $8 purchase.