Investing is cheaper than ever. Trading is free, some index funds may as well be, and a diversified portfolio can be built by machines for a fraction of the cost of live professionals who deliver advice in an elegant leather binder.

In the space of a few days last month, the price war among the brokerage firms pushed the cost for many trades to nothing at Charles Schwab, TD Ameritrade, E-Trade and Fidelity. Then, this week, Schwab said it would acquire Ameritrade for $26 billion — a deal that demonstrated the importance of market share in an era of cheap investing.

But low-cost investing isn’t always as cheap as it appears. Many companies, in stamping out certain fees, are doing other things that can cost you money — and it’s up to you, dear investor, to figure out what they are.

Each firm’s policies differ, but here are helpful places to look: the way your brokerage uses your cash holdings; the costs of other services it offer s; and how it might be profiting off your free trades by getting someone else to pay for them instead.