Ral Zarek is a solid planeswalker with three abilities. His first ability adds one loyalty counter to Ral and both taps a permanent and untaps another permanent. The second ability costs 2 loyalty counters and deals 3 damage to a target creature or player. These are both solid abilities. The last ability costs 7 loyalty counters and allows you to flip 5 coins. For each of those 5 coins that lands on heads, you take an extra turn. It's a very powerful ability that can win you the game in the right deck.
Because of his powerful ultimate ability, Ral Zarek sees some Modern play in rogue Doubling Planeswalker decks, due to his strong interaction with the enchantment Doubling Season. That Enchantment allows Ral Zarek to come into play with eight loyalty counters, meaning that he can use his ultimate ability to take extra turns right away. In Commander, however, the deck he sees the most play in is the massively popular Narset, Enlightened Master.
In 2018, you can find copies of Ral Zarek listed around $7 to $8 USD. Various Magic card vendors will buy copies of Ral Zarek for as much as $4 in cash on their buylists, which is a fair price for a mythic rare card that only sees play in Commander. The true market price of Ral Zarek is closer to $6.75, according to TCGPlayer.
Ral Zarek is a solid planeswalker who is a solid addition to any Magic the Gathering card collection. He is a card with solid value and a price that has been relatively steady. Barring a reprint, Ral Zarek should be a solid investment for your deck building toolbox or trade binder.