It's a good idea!
cons: depending on how you buy your BTC, the current minimum tx fee will be the downside
it would cost you (or seller) significantly high when dealing with low amount transaction
if you can find a way to reduce the fee close to negligible, it would be perfect
Atm I just registered on Coinbase and waiting for verification from there, I thought of buying on monthly basis. Is it a good idea?
Coinbase has fairly high fees as well. Take a look at GDAX. It's owned by Coinbase and the fees are considerably lower. Don't forget to pay attention to other exchanges as well. Gemini is a good US based exchange with far lower prices than Coinbase (although higher than GDAX).
Don't forget to get your own wallet, or if that confuses you start researching what's involved in having and using your own wallet.