I called MS today.
First off, they are old school. They are only interested in serious investors, and to open an account, they prefer if you go down to their office to do so. (As a matter of fact, I don't know if there is any other way - perhaps by mail?).
They put you in touch with a personal account manager, and he or she opens the account for you.
To my knowledge [i.e. what they "told me"], the only fee that is accrued [for a "Brokerage account"] if you have under 250k in your account is 1 1/2 percent of your account balance. So if you have 10k in your account, your yearly fee is $150. (And, presumably, if you have 1k, your fee would be just $15/yr)
This is all I have been able to get out of them, short of going down to their office to find out if there are any other "hidden fees".
However, this is what the agent told me, and may not necessarily reflect the ultimate truth. (Maybe other "hidden" fees?) Another question I would have is, if I open the account as a joint account for my wife and I, can I then get two platinum cards, one for each of us? Or would Amex say "one per account"?
If anyone has more accurate info and can shed some light on this, it would be beneficial for quite a few users here.