Whimsicott #148 (Pokemon Cosmic Eclipse | Pokemon Cards) prices are based on the historic sales
The main difference between the two is Greninja is a Stage 2 that can hit any of the opposing 'mons, even any weakened ones on the Bench
Any card (except for banned ones) from the most recent sets will work
However, while Ruric Thar is heavily dependent on the spells being cast by your opponents to be of specific types, Omnath's ability is based off of the cards that you cast, putting its triggers entirely within your control
In short - yes, these Pokmon collectibles cost money, but not every one of them can bring the owner a few hundred bucks
If your strategy revolves around Auras, Winds of Rath is the ideal card to pick it will protect your creatures from removal, and often end up being asymmetrical