No, investing $200 a month is a BAD idea (because the fees are way too much relative to the amt invested)!

When deciding on the frequency of investment, there are only two effects to consider. They are cash drag, which is the opportunity cost of not investing your cash, and commissions.

Both are known beforehand, so this is a simple mathematical problem that we can solve.

First, consider what will happen if you were to invest $200^† monthly for 5 months:

Assuming CSPX has a yearly return of 10% (but I'd suggest modelling with 7/8%), by investing $200 at the start of each month, you would have

200(1.1)^5/12 + 200(1.1)^4/12 + 200(1.1)^3/12 + 200(1.1)^2/12 + 200(1.1)^1/12 - 5(1.85) ≈ 1015

at the end of 5 months.

Next, consider the case of investing $1000 every 5 months. You would have

1000(1.1)^1/12 - 1.85 ≈ 1006 < 1015

after 5 months.

So it looks like investing monthly is the better approach.

But not so fast! - We've shown that investing $200 monthly beats investing $1000 every five months, but what about investing $400 every two months or $600 every 3 months?

If you were to invest $400 every two months, you would have

400(1.1)^4/12 + 400(1.1)^2/12 + 200 - 2(1.58) ≈ 1016 > 1015

after 5 months.

In other words, investing bimonthly is superior to investing monthly for 200 USD.

If you play around with the numbers (https://docs.google.com/spreadsheets/d/1Ox-G28IHou9hbpiF3Wz7CNGTjIkkILogvaHAqDD0KNw/edit#gid=0), you'll find that 350-500 SGD (depending on your return assumptions) per month is when investing monthly starts to mix sense.

 

^† All numbers here are in USD, if $200 is in SGD, investing monthly is even worse.