If you find a bundle you like just get it. The good ones never repeat, but cheap MOBA things will crop up frequently, and its limited supply. They only have so many keys to give away.
There's no real guarantee about the "cheapest" price for new but the black friday is your best bet that someone will offer an nice deal. The only thing cheaper is desperation sales if a particular card isn't doing so hot, but that isn't happening with any of the current cards judging by their persistently high prices.