Which brokerage would be most cost efficient if I want to invest in S&P 500 over the next 20 years?

To add on to what @NicholasBeh says...

Not to mention that once you hit StanChart's priority banking, you can further enjoy no minimum brokerage fees. So you can literally buy $100 worth of shares and be charged $100*0.2% = 20 cents (or 18 cents if you buy SG stocks) for brokerage fees.

StanChart's priority banking is also one of the easiest to get into at $200k in deposits/investments. ETFs are also considered as an Asset Under Management (AUM) by StanChart, unlike some other priority banking services that only recognise their own Unit Trusts as AUM.

While priority banking might sound far-fetched, with discipline and diligence, $200k is definitely attainable by a working adult in 8-10 years.

- Jeffrey (jeffreyleezq.com)​​​

I would recommend you to use Standard Chartered. For your use case, commissions should be around 10 USD per transaction and there will be no recurring costs for holding with them. Their spread on foreign currency conversion can range from 0.3-0.7%.

In the future when you increase your investment interval and frequency (e.g. $1000 a month), I would suggest that you switch to Interactive Brokers when the savings in foreign currency conversion starts to outweigh the monthly minimum commissions that they charge.

I would not recommend using Saxo as they charge a relatively high 8 GBP minimum commission for LSE. They also charge a 0.12% p.a. custody fee, which I find particularly problematic as it is a recurring fee that will be charged as long as you hold shares in your Saxo account.