Which is cheaper to DCA $200 per month into ETFs over 20 years, FSMOne RSP or Tiger Brokers?

For $200 a month, you'll incur the minimum for both. So for Tiger that'll be USD 23.88 (plus taxes) and for FSMone, that'll be USD12 (plus taxes).

Note that for selling on FSMone, you'll incur their normal selling charge (Min USD8.80) but if you're long term then no issue.

Also note that both incur the 30% dividend withholding tax since you're purchasing US ETFs.

I would say both these options are feasible and are quite low cost already. That said, on $200 a month, these comms are about 1-2% of your investment value. As such, you might want to scale up your investment (if possible) or DCA less frequently (under tiger) to maximise the comms.​​​