SWF - General Discussion, page-28

Target them at the cost of: reduced/lost revenue on trades <$2000, from existing users.

They'd unlikely be a loss leader on the pricing, so minimal additional costs (assuming they don't go crazy and spend lots of marketing on the small-trades users).

Benefits: break even brokerage at worst on the small trades, small profit on the cash balances, higher market share and brand recognition, higher organic growth in more valuable segments (from word of mouth etc), retaining the users as they age and become more wealthy.
Disadvantages: support staff costs, more bumpy user numbers as they quit for the next cheapest thing on offer, lower revenue on small trades from existing customers.