The interest rate is a function of the utilisation ratio: total borrowed by all users divided by total deposits of all users. The interest rate will increase slowly approaching a utilisation ratio of 70% but will increase rapidly above that to encourage the market to deposit more funds.
Does it cost anything to open a Mango Markets Account?
Yes, to cover costs of creating necessary accounts on the Solana blockchain and storage.
The costs is 0.032 SOL to create a Mango Markets Account and also 0.025 SOL for each new spot market they trade on.