Considering Saxo fees (0.12% p.a on monthly for custodian fees for ETF) and Saxo trade comm min per trade of USD4, how much should I invest at a time so that it would be cost efficient?

From the info you have provided, I will assume that you will be investing an amount of $1500 quarterly (currency in SGD unless specified), with the possibility of investing in Irish-domiciled US ETFs listed on the LSE for the lower 15% WHT. Currency of the ETF will be in USD.

I will compare between 4 brokers - Saxo, Standard Chartered, Interactive Brokers and Tiger Brokers. Do note that Tiger Brokers does not support LSE.

Fee summary:

Cost per transaction:

As you can see here, Interactive Brokers comes out top for having the lowest commissions. However, since you are investing quarterly instead of monthly, the monthly minimum commission of 10 USD would make it more expensive than the other brokers. Saxo is fairly cost prohibitive as well, and it does not include custody fees which will grow as your investment value grows.

Hence, for US-domiciled ETFs, go with Tiger Brokers. For Irish-domiciled ETFs, go with Standard Chartered. And if you switch to monthly DCA, go with Interactive Brokers.​​​

DCA about 1.5k quarterly to etfs