The Steem Monsters Starter Pack is a $10 pack that contains 30 Steem Monster Cards. It contains a nice mix of cards with all the different basic 3 Mana summoners from the four main Splinters, as well as minions to play all of these Summoners. It will contain both Common and Rare cards, but it won't give you any Epic or Legendary cards. It won't give you any gold cards either, so you need to open cards packs for these.
You can buy the Starter Pack with pretty much any cryptocurrency you want to, and it even includes a free Steem account if you don't already have one. You are also free to sell any cards from it that you do not want to keep, so it's possible to reduce the cost by quite a margin if you only plan on playing with a certain Splinter.
If you want to play Steem Monsters, then you will need to buy a Starter Pack, even if you have a lot of different cards on your Steem account already. This was done to prevent people from making lots of different accounts to farm the rewards from daily quest, so you can think of the Starter Pack as kind of a game license that is required to play.
However, one you have bought the Starter Pack, you are free to play the game as much as you want forever. There is currently no way to play the game without a Starter Pack, so you will need to shell out $10 if you want to play it.