As for why the card sold for so much money, the most important reason whether or not it justifies its price is another more subjective question is collecting
The Japanese PLAY promos required 70,000 loyalty points to obtain in 2005 and are the most valuable Gold Stars on earth, several times their English POP 5 counterparts
Thurid, Mare of Destiny is a pegasus that belongs in the section below as well, and is a straightforward payoff for playing lots of unicorns
Muddle the Mixture Transmute is an ability that lets you discard your card to search for something with equal mana value
Its also a pretty decent protection spell or a wrath, depending on what you need
You can still play expensive spells that cost 6+ mana, it just means you should avoid running too many without a clear plan on how you will cast them