how mutch we need to spend to join MEW with Metamask?
You don't need to spend a dime to join MEW with Metamask! Why do you need to make an account in MEW or Metamask? Those are just used for viewing and spending your wallet balances and all the coins are stored in the blockchain. Assume it something like GUI used to communicate with the eth blockchain. You can create your own wallet without using a MEW, by just using a dice or by flipping a coin. Private keys are generated which further can be used to spend the coins. Consider the private keys as the key for opening your room of fortune. These private keys can be used up in any trusted eth wallets to retrieve your coins or spend them. Metamask acts as a bridge between your eth wallet and the decentralized apps. They can also be used as wallets. You can download the metamask from chrome web store and can integrate your created eth wallet with it.
can you tell me a good exchange only for erc20 tokens?
ERC20 tokens are listed in either a decentralized exchange commonly called as DEX or a centralized exchange. A decentralized exchange is one in which you hold the private keys of your wallet and the trades occur between peer to peer without a trusted third party. You can just integrate your wallet in the exchange. Some of the popular DEX exchanges are IDEX, Forkdelta, Cryptobridge.
A centralized exchange is one in which your funds are controlled by a third party and the trades never occur peer to peer. They are easy to use but when hacked leads to the complete loss of your funds similar to the Mt.Gox. Some popular centralized exchanges are Binance, Okex, Bitfinex, Huobi.