How much is a good minimum amount to invest in monthly STI ETF?

A good amount would be an amount you are comfortable without each month. I.e. you wouldn't affect your livelihood!

It depends on your income since how much you invest is usually calculated as a percentage of your take home pay. If you are following the suggested budgeting ratio, there should be also sufficient diversification between asset classes with respect to your age. Most ETF has minimum of $100 monthly, but it's hard to advise a suitable sum without much information.