The interest rate is a function of the utilization ratio: total borrowed by all users divided by total deposits of all users. The interest rate will increase slowly approaching a utilization 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 account?
Yes, to cover costs of creating necessary accounts on Solana blockchain and storage.
The costs is 0.032 SOL to create MangoAccount and also 0.025 SOL for each new spot market they trade on.