RAM is required to interact with contracts and in this case to mint NFTs. You will need at least 151 bytes of RAM to mint an NFT. You start with a little bit of RAM reserved to interact with some contracts if you created a WAX Cloud Wallet, which can be used to mint a few NFTs. The value of RAM fluctuates frequently, to check the price see https://wax.bloks.io/. The cost of minting one NFT on WAX currently is around 1¢.