There are so many cards of four-cost or less that can utterly change the game when they hit the field, and Beseech the Mirror just gave you easier access to any number of them
However, fixing power level via rules changes is something Wizards avoids these days, largely because of the ridiculous saga of Time Vault
The creature itself gets sacrificed if it isnt cast from the graveyard by exiling other cards, but this is hardly an expensive cost since the things it does can fuel its own return
The bulk of the $200 price difference comes in the form of Collector Boosters, though they added non-card collectibles, like a binder and art print, as well
Free spells are very, very powerful, and Id argue its between this and Fierce Guardianship for the best in the cycle