On a normal day if a player suddenly decided they needed a Crystal Unicorn, a Grand gem grove and 20,000 apples, the payment in gems would equal 79.99. In that respect 19.99 is a very good price if not quite as much fun as winning the items.
The concept of a value pack being "worth it" has more to do with considering the amount of time you gain enjoyment from game, the things that would make your game play more enjoyable (having a crystal unicorn benefits you in coins, battles and breeding) and the choice to pay the developers with some income in exchange for an item you'd really like to include in the puzzle that this free game can be.