There are often questions about using SCB vs IBKR, and what if buying monthly/bimonthly/quarterly... I tried to model the costs of the commonly discussed options in a spreadsheet -- no idea if it will make sense or be useful to anyone else, but here it is:
https://docs.google.com/spreadsheets/d/1BqIhbYNJF7VJbtj6PS08Csbyx1s4vs_03Kvh0pPRUCI/edit?usp=sharing
I'm sure someone will point out if there are any glaring errors
I did find it a bit surprising that while it makes sense to use IBKR over SC if you're intending to buy $700/mth of IWDA on monthly basis, SC is still cheaper up to $1800/mth on quarterly basis. I'm not sure how to weigh the value of clocking in more time-in-market, to decide between investing monthly over bimonthly or quarterly.
(You'll have to make a copy for yourself to plug in your favourite scenario. Click on File -> Make a copy.)