It unlocks the 30 locked cards in each deck, not a new deck. Therefore it saves time not money. The keys have always been a bit of a rip off. They lock so many cards that you'd have to win 300 matches to unlock all the cards - so it's either pay for the
keys, play with a gimped deck or spend 100 hours grinding for cards.
In a couple of months, they will also add more advanced decks which you will pretty much be forced to buy unless you're planning on only playing against the AI.
Don't waste your free key on foiling a deck, the effect is not great and most competitive players turn off the foiling in the options.