To expand on that point, once new crates are introduced into Battlegrounds, you can purchase them with your in-game currency, though each crate will require a key to open, costing you $2. 50. However , cosmetic items do not affect your ability to play the game, and there's no actual requirement for you to open any crate yourself.
PlayerUnknown's Battlegrounds cosmetic items are currently open to buy and sell on the Steam Marketplace, and just as with Cheap CSGO Skins, it's more than likely that most Battlegrounds cosmetics will fall below the $2. 50 key price.
Unless you're aiming to open a cage for extremely rare items, which are likely to cost more than a key itself (not to mention subject to RNG), you can simply buy the item from another player within the Marketplace.
Similarly, Bluehole has yet to reveal any sort of item rarity system that would make Battlegrounds skins wildly expensive. Yes, there are expensive skins around the Marketplace that'll cost you around $300, but those were the cosmetics included in the PLAYERUNKNOWN's Set, Buy CSGO Skins and were exclusive pre-order purchasers.