There's an excellent data analysis post by u/IamMandrell on reddit, analyzing trophy decks (that's to say, only looking at decks that reached 7 wins)
Later, a 4-mana investment gives you an Archaeomancer-like effect, so you can get whatever you need back
Meowth ex Ability: Trump Card Catch Once during your turn, when you play this Pokmon from your hand onto your Bench, you may search your deck for a Supporter card and put it into your hand
If you -3 her you can steal, for a turn, a 'walker or dork, untap it, and it has haste
Even those that have real cards on the back dont tend to have big mana value differences between them, so the benefit for transforming isnt massive
The most expensive card in the Silver Tempest set is Lugia V Alt Art